Why one and not the other?

Probably it is the predilection of northern europeans vs western asians.

In Buddhism everything is suffering.
In Christianity everything is about original sin.

The only thing really better about Buddhism than Christianity is that they preserved the words of their great teachers far better.
Christianity had several cycles where they modified the bible. Naicia, King James...

The other big thing is that Buddhism is not a religion. It is a path of following the Buddha's footsteps.
Or a pathway to godhood. (in Christianity it would be termed, becoming like Jesus)

In the future, all the religions will crumble.
As we are seeing Catholicism crumble now. (even more so after information comes out about how they have lied to all the people, and how badly)

In the future, we will have people who are spiritual, and seek spiritual enlightenment. They will use all the great texts, and not be beholden to any religion.