We all know Dark Souls was a hit, but maybe From wasn't quite sure they would be able to capture lightning in bottle twice. So they played safe and signed another deal with Sony.
When you think about it, that worked well for them. DS2 wasn't as good as DS1, but Bloodborne was HUGE.
It was Sony that went to Fromsoft to have a game made, essentially. It was going to be a Sony game from the start, and if they hadn't done it, we wouldn't have Bloodborne at all.