Because they wouldn't exist since everything in history good or bad has came all the way up to your existence. If you changed history as major as christian influences you change the outcome of people in the future. It is not like you have a lotto ticket waiting to get into the human body. The only thing this exhibit shows is that maybe we would be more technologically advanced and that is hoping people don't latch on to another religion. If Hitler didn't rise to power and war refugees didn't flood to America you might not exist today. Not to mention resetting man kind seems stupid and has no beneficial gain unless human kind was at the verge of extinction.

it would ensure they existed sooner.

Life doesn't work that way.

If i had a human in parallel universes. Name A & B but A=B

So A is a African American with half white blood from his mother escaping unjust laws from her home country and coming to America. Now, B is a pure African who did not have these unjust laws because some time traveler changed the laws.

Now A does not equal B. A's personality, believes, or desires, goals are not B's and can be said to be a different human being. Same can be said about B being different from A.

Fast forward 2000 years and population has changed due to A becoming B, those who would have lived as A's descendants do not exist (all from a small change) since we changed A into B.

B now has descendants but all you have done is decide who should exist and who shouldn't exist. Not to mention new factors that could make history worse then it is now. You couldn't see the outcome because you reset history with a new point of alteration. So now you are hoping that this change is for the better of no evidence that it will make it better and are throwing away everyone's current existence for a re-roll even tho we at this point in the future are smart enough to make time travel.

Now to me, I don't feel like i have the right to determine who exist and who doesn't. Especially the scientific data that says that human population has a high chance of destroying itself so by the time we get time travel i think we are going to be civilized enough not to wipe ourselves of the face of the earth.