It's universally accepted that murder and assault are crimes, so the culture does not defend perpetrators of these activities. When someone makes a joke about killing people, everyone listening is aware that the person doesn't condone, or isn't looking to explain away, murder.

However, rape victims still face a lot of prejudice, and in many cases, they're blamed for their own rape because of the way they dressed, or because they went out at night or left their drink unattended or whatever. Rape victims are not sympathized with, the way victims of assault are, so these kinds of jokes that paint rape as something to be laughed at make it even harder to report. Most jokes about rape normalize prejudice against victims, and further diminish the seriousness of rape. Why is rape an appropriate punchline?

To be fair, I don't think that rape should be completely off-limits to all comedians; I just think that as the topic becomes more sensitive, the joke really needs to be well crafted so it remains funny but doesn't underplay the seriousness of the situation. Jokes that laugh at rape victims or treat rape as a punchline aren't funny and can be actively harmful; they just enforce the cultural status quo that tacitly tolerates rape. Jokes that are focused on the perpetrator or the wider culture that blames rape victims are subversive, and these are the really insightful jokes.