As far as the PSP is concerned, three major Enix titles were remade on that System (Valkyria Profile and the first two Star Ocean games) and the other major Enix game from the PSone (Dragon Quest VII) has been remade as well, as have all the SNES DQs and the NES DQs. So in this case, this might have to do with them considering these versions as superior or just not wanting to bother with creating a wrapper for PSN classics.
Other titles seem to be in a strange state of who actually can release them, most prominently the games by Quintet, whose rights remained with that company and the publishing contract may have just run out. Some games, like Crystallis, have been released though.