Because it's called "Christianity"? As in, based on Christ's views and beliefs? And these pretty much outright contradict all the sick fuckery that's going on in the Old Testament?

Myself, I'd prefer it if Christianity rejected the Old Testament altogether. It's an awful book and it should never be a basis for a peaceful religion.

(I'm an agnostic atheist, btw, but a former Christian who has complex feelings towards Christianity).