Why does mass bend space?
I have a feeling Einstein knew, and perhaps even told us, but we haven't quite understood yet.
I think it's more about space-time being bent, rather than space alone. I'm not being picky, I think it's key.
All mass is attracted to all other mass very weakly, (and that, apparently, is gravity) unless there are reasons for it to repel, such as a magnetic field. two lumps of metal in space will attract, while the same lumps magnetized and placed like pole to like pole will repel. So then, perhaps the very weak attraction is actually the same force as the magnetic force and the weak electrical force.
Even if that isn't all rubbish (and it almost certainly is) it still only moves the why question to another place.