The Vita was too powerful for its own good; everybody quickly assumed (because Sony marketed it as such) that the Vita would play slightly sub-PS3 games. When those games never materialized, nobody bought it, and with no install base, Sony wasn't going to put that level of effort into making the games people expected. That's the only answer you really need, but if you want my personal one, I'll give you a twofold reason: Every game I wanted to play on Vita, with no more than two exceptions, is playable on the $20 PSTV or on PS4. The only notable game I can't play outside of Vita is Golden Abyss, which I'm fairly confident will end up ported sometime soon.
You should check out my YouTube channel, The Golden Bolt! I review all types of video games, both classic and modern, and I also give short flyover reviews of the free games each month on PlayStation Plus to tell you if they're worth downloading. After all, the games may be free, but your time is valuable!
