Why is a doomsday so important to Christianity?

Because they are terrified of death. I mean that's what all religion is it's the promise that death isn't the end. You die but then you wake up in this new life and this one is forever. But even that is not enough because even if they think they are getting a new better life on the other side, they STILL don't want to have to die and that's what the rapture is. A byp. It let's them skip the dying part and go straight to the afterlife.