Its just not worth the money for them, especially in the situation they are now. Porting their games to PC is just infinitely more lucrative and safe. Its kind of annoying that this Vita situation is brought up in every single Square Enix thread as of late. Its way more understandable in Type-0 threads, considering it was originally a Vita game, but its doesnt take much really to perceive why its not being released in that platform in the west.
Like Wii U, you're really only getting these platforms for exclusives. It seems every major multiplatform title is guaranteed to be on PC and that list is only expanding, and it's really hurting the image on why people get consoles in the first place. It's now becoming more of personal convenience to get it for a platform you don't have to upgrade and less of it having games not found elsewhere. Sans exclusives, of course.
If this is true, I really believe Final Fantasy XV will eventually be coming to PC. It's a no brainer after their last major FF projects now having PC releases.