Some people saying mobile porting is the reason but I have to disagree on that. GTA just flat out moved to home consoles. One of the highest selling series on the consoles (MH) moved to 3DS completely while some of the stuff Sony made (Daxter etc.) just disappeared or went full on to home console. SE's stuff like Crisis Core and Dissidia (both amazing games) would be hard pressed to work on mobile devices due to so many button inputs (unless you're going to heavily overhaul the button inputs). They would honestly have better luck in porting stuff to consoles like they did with Birth by Sleep.

In the end I feel like there really is no reason to dip into the PSP library because they're really is no reason too.

Basically most PSP stuff is available on Vita. Chuck in mobile ports too and I guess they decided there isn't much market left with the 3DS where the audience generally buy the console just for Nintendo content.

I think the Vita would probably have done better if there was still a market on dedicated handhelds for this stuff.

The most common answer is that 3rd party developers have a hard time competeing with Nintendo first party titles. There are a limited amount of games most people afford to buy (an attach rate of ten games per console is considered good). Of the 37 most sold 3DS games only 8 are from other publishers than Nintendo (Monster Hunter, Yokai Watch, Dragon Quest, Kingdom Hearts, Puzzle & Dragons).
Shin Megami Tensei has a hard time selling on 3DS despite good reviews and frequently occurring sales (€10 digitaly in Europe).

To be fair, the numbers of JRPGs are usually great everywhere but Europe. We don't have here a big culture of JRPGs outside of the bigger ones (FF, Pokemon...).