In order for your argument to have merit, we need to establish high profile games that allow cross platform across PS and Xbox and PC ecosystems. Cross platform games between Xbone, PC, and PS4:
Farming Simulator
The Golf Club
Those are the main games that allow it.
The Division was originally supposed to allow cross platform. But the developers backtracked and it doesn't anymore.
You are highly underrating cross platform, I think.
Name a single cross platform high profile game that allows you to play on the big three between platforms: PC, PS, and Xbox besides obvious stuff like FFXIV (which funnily enough, came out when MS was willing to find Japanese exclusives for their 360 platform). Now that MS is the runner up in the race they have zero reason to give Sony any rope.
We are not at the point where cross platform between all of the big three is a thing yet.
But maybe it is Sony's deal after all. Who knows? We don't know. All we can do is guess, but you shouldn't underestimate MS' willingness to be petty as fuck. That's kind of their thing.
Also, PSN is designed to allow this. Xbox Live, not so much.
More than likely, given all evidence, it's probably both: Capcom wanted cross platform across all platforms for SFV. MS said no. Sony paid exclusivity then. The real question is whether or not it'll eventually come to Xbone. I really, really doubt it. This doesn't seem like a Tomb Raider situation.