For some presumably technical reason, Vita's PSP emulation is imperfect. That's why new firmware had to be introduced to allow PSOne Classics – which are PSOne games in a PSP wrapper. And that's also why not all PSP games are immediately playable on Vita. Sony has never officially explained this, so I can't be more specific than "some technical reason."
But there's no explanation for why some games work, but aren't on the store. Unless Sony is deliberately trying to roll out games a few at a time to maximize sales of each round of games, there's no conceivable benefit to having games available, but only through complicated workarounds.
I suppose Sony could be doing separate quality assurance on each PSOne game to make sure it makes the transition well, and only releasing them on the Vita PlayStation Store when they're verified by QA. But if that's the case, then all these "side-loadable" games represent potentially untested software, which is the kind of thing that leads to crashes or, worse, exploits. I'd expect Sony to stay on top of verifying everything that can run on the system, especially from official sources. On the off chance an exploit is discovered, games will mysteriously disappear
from PSN, and we lose the chance to play Symphony of the Night
That leaves the possibility that Sony just doesn't know that these other games work, which puts users in the odd position of being voluntary QA (by discovering that they're playable) and voluntary marketing (by telling others about these unannounced compatible games). And users who don't frequent forums and video game news sites like ours, the poor wretches, are left totally unaware, and subject to a selection of 13 PSOne Classics.
In either scenario, something needs to change. As it is now, games are arriving on the platform in a random, scattershot manner, and Sony is telling customers about only a few of them. This would be unacceptable on any other store, in any other medium.
"You can only watch Austin Powers 2
from our service on certain devices at launch," I could never imagine a Netflix executive explaining. "But if you start another movie on your computer, pause it, and then start up Netflix on your phone, there's a chance it might work." Wired's Chris Kohler imagines iTunes on a PSOne Classics model
, where a few of your purchased songs are released for use on your new iPhone periodically. Not only would these scenarios be unacceptable, they're literally unbelievable – and that's how PSP games and PSOne Classics work on the Vita, now.
It takes some doing to turn "making more games available" into something that angers and confuses the people using the device.