TL;DR first: To prevent Jesus from being crucified would deny God what he wanted most: to be made right with the creation he loves so much. To prevent Jesus from being crucified would prevent our entire world (the people, the planet, everything) from ever hoping to one day be made right again. But through Jesus God won back his creation from Death and we now have a hope that one day all things will be made new. Why would I stop that?

For a more in depth response or clarification for all that crazy talk here ya go:

Christianity has become so preoccupied with simplifying the story of Jesus that they have reduced it to some guy 2000 years ago "died for my sins." In its most simplistic form, I suppose that works, but it really disregards the full story.

I'm not looking to convert or convince anyone here. This isn't an argument for Christianity, rather how going back in time and stopping Jesus being crucified wouldn't fit within the greater Christian framework. In short, it's bad theology.

If you start in Genesis you have a good god who creates a good creation. This is the Hebrew narrative, which flew in the face of most (if not all) mythologies of the time. Take the Babylonian myth in which the pantheon of gods deceive and war with each other. Marduk kills the evil Tiamat and fashions the earth from her and humans are created as slaves to the gods.

The Hebrew god chooses one of his creations to bear his image, to give a creature the ability to think and love in the way God does, in a way no other animal is capable. Humans have right relationship with themselves, other humans, and God. In Genesis, creation is good and humans—God’s image bearers—are told to procreate and "subdue the earth” (Eden was not a resort, there was work to be done).

At the Fall, you have a story in which humanity (represented by Adam) is convinced that God is holding out on them and decides to rebel. It turns out God was holding out, he had given them all of the goodness and had withheld the badness. Their actions brought sin into the world, bringing with it death, decay, pain, suffering, etc. (embodied for our purposes as Death, with a capital D). Our image bearingness was fractured. The first thing that happens is Adam and Eve realize something is wrong with themselves (shame from nakedness). They then blame each other when questioned by God. Then they realize that, because God is perfect, and they are no longer perfect, that their relationship with God is broken.

Theists often have a hard time answering the Problem of Evil, but one thing is for sure, the Christian God was not idle regarding it. He set out to make it right. He chose a group of nomads, hunter-gatherers, to be his people. He made a Covenant, a promise to Abraham, that the world would be made right through his descendents. As this people group grew, God gave them the Law, the Torah, a way to live. These people soon became 12 tribes, later became a chiefdom (rule by judges), and later retook the land where Eden once was and settled there in a city-state. God gave them a king and they were the nation of Israel.

I just covered a lot of history, but it's important to understand that during this time God dwelled among the Hebrews, but he was holy, which means "set apart." Because he was perfect and humans were not, he could not be directly among them. For this reason he dwelled in the Ark of the Covenant during conquest and when the temple was built his earthly presence dwelled in what was called the "holiest of holies" a portion of the temple separated by a series of chambers and finally a very thick curtain. It took a lot of ritual cleansing for a single high priest to enter.

The nation of Israel underwent four major occupations (as foretold in Daniel, regarding the four beasts). The king or the people would often displease God and he would basically take his hands off the situation and they would be conquered. During this time, there were promises through prophets that God would give Israel a king that would make things right. Most Israelites took it to mean king would militaristically rise up and conquer the world for Israel.

The fourth occupation of Israel was under Rome. The Israelites still hoped for this king, this Messiah, to come deliver them from foreign oppression and one day a rather charismatic guy named John the Baptist started saying that the "Kingdom of God" was near. This language fit with what had developed to Jewish eschatology (belief about the end times, developed through prophetic texts like Isaiah, Ezekiel, etc.). God would establish his kingdom on earth through Israel and in the last days there would be a bodily resurrection.

There had been plenty of people claiming to be this Messiah. They all either lost momentum, were quickly killed by the Roman government (as to claim kingship opposed Caesar), or were exposed as frauds. Jesus was not the first person to come along claiming to be this Messiah. But his message was a very peculiar one. The Jews believed that by being Jewish, by their ethnicity, they had been given the Torah (God's Law). By simply possessing the Law, they were God's people. Many, particularly a Jewish sect of theologians and Rabbis (priests) called Pharisees, held strict standards for following the Law. They were the fundamentalist of the first century.

It was rather curious when a great rabbi named Jesus started claiming to be this Messiah. Yet, he didn't always follow the Torah, the Law, in the way they wanted. He did all sorts of counter-cultural things like love sinners, forgive prostitutes, heal on the commanded day of rest, respect women, care for the poor. When he loved sinners, they would ask how he could do such a thing. He would point out that they were sinners too. Just because they kept the Torah on the outside, didn't mean they were keeping it on the inside.

Jesus also claimed was that he was God's son, who was also one with God (we're not going to get into trinitarian doctrine here, suffice to say that Christians believe in three persons of what is one god). Jesus claimed to be able to forgive sins. Well, you can't be the most loving person ever and do a lot of miracles without getting some popularity. He was well liked, but Jewish leadership thought he was a heretic and one thing led to another and he was executed.

At this point, his followers thought they had bet on the wrong guy. He was just another false messiah. But three days later, the most peculiar thing happened, the man Jesus, who claimed to be God the Messiah, came back to life.

Why wouldn't I stop the crucifixion? Let's apply this history to the life, death, and resurrection of Jesus. The Jews had a complex sacrificial system, so the idea of God dying for their sins, of being the perfect sacrifice would have resonated. Why was this necessary? Humans brought Death into the world, God came and in the perfect sacrifice undid Death. Jewish theologians were looking for a king who would defeat their physical enemies, the kingdoms that held them captive. Yet, as the Jews believed that in a future age there would be a bodily resurrection, that the effects of the fall would be undone, they had here a person claiming to be their Messiah who defeated the ultimate enemy: death.

Through Jesus, our broken status as God's image bearers was made right. Jesus was the perfect man that Adam couldn't be. He was perfect king that Israel couldn't have. His death made it so that the broken relationships that humans have with themselves, other humans, and God could be redeemed.

Though humans could not keep up their end of the deal, although Israel failed God again and again, God upheld his promises and gave them the king they really needed. He didn’t give them a king with military power, he gave them a king that could deal with the real issue. He could deal with their hearts, he could deal with the brokenness in the world, he could overthrow the power of Death.

Jesus spent around 40 days on earth after his resurrection. He taught that his actions didn’t just redeem Israel, but that God had—through Jesus—made all of creation right with God. To be Jewish and to have the Law wasn’t good enough, because the Law couldn’t deal with a sinful heart. Jesus was not merely the king of the Jews, but rather king of all creation.

The age of humanity’s separation from God had come to an end, but the last days, when all will be made right have yet to come. We are in between those ages. New life is being born, but it’s not quite birthed fully. Jesus’ resurrection was the eschatological event—the event that was supposed to happen at the end—that was brought to the middle of history. In doing so, the message is not just reserved for a certain people group, but for the whole world. When Jesus prayed that God’s “will be done on earth as it is in heaven” he literally meant that the perfect world that God lives in it will break into earth. It means that God is presently at work and that there is hope in the darkness. The temple curtain has been undone. God now dwells in and among us. That’s the Gospel, which translates “Good News” and we are to tell others about it, not so that they can be in a club or have fire insurance, but so that they can participate in this God’s coming kingdom of hope and healing and love and right relationship.

To prevent Jesus from being crucified would deny God what he wanted most: to be made right with the creation he loves so much. To prevent Jesus from being crucified would prevent our entire world (the people, the planet, everything) from ever hoping to one day be made right again. But through Jesus God won back his creation from Death and we now have a hope that one day all things will be made new. Why would I stop that?