I would say you started out with a faulty premise... That Christianity only existed because Jesus rose from the dead.
Because clearly telling people to love their neighbors and treat people the way they want to be treated only has resonance if a Zombie tells you to do it. Apparently.
When in fact, it's quite the reverse. If Jesus existed at all, the miracles, including the resurrection, were tall tales that got bigger with each telling.
The thing about the New Testament is that it isn't arranged in the order it was written. The first books that were written were the epistles, which don't talk about the resurrection, the virgin birth, or the miracles.
Then you had the Gospel of Mark, which is actually kind of laid back, and in it's original form, only had an empty tomb, no resurrected Jesus.
The Gospels of Matthew and Luke came next, both of which were heavily borrowing from Mark, but the miracles come fast and furious.
Finally, you have the Gospel of John, written last, with a lot of the supernatural stuff in it, like Doubting Thomas and such.
So the question should be, "Would Christianity exist if Jesus didn't rise from the dead" and should be "Why did the people who created Christianity need a resurrection to sell their story?"