As it's been said, it's a simple matter of the size of the user base, more importantly, relative to the amount of time on market.
8-9 million sold over a two year period, give or take simply isn't enough to justify diverting AAA resources into titles, regardless of quality, that are only marketable to such a limited audience.
I say this as a consumer who pre-ordered a Vita, loved it, still consider it to be the best portable gaming device I've owned over decades of gaming, but with all the other portable options available (namely iOS devices), I'd be lying if I said the platform had my full financial support as a consumer; I just don't buy a lot of games for it, and when I do, it's usually because they're on sale.
If anyone is going to pour AAA level resources into a title, they'd be mad not to do so on a PS4 game instead.







