The only series that sold better than Phantasy Star on PSP were Monster Hunter, Final Fantasy, Metal Gear Solid, and Kingdom Hearts.
After Phantasy Star, God Eater got very close, and then we had AKB, Gundam, and Hot Shots before we hit Super Robot Wars.
For the most part, it looks like the system has the PSP's medium tier sellers, but is currently lacking the high tier and the megaton tier.
We know Kojima's next title is at best multiplatform for the system, so that likely hurts there.
With Kingdom Hearts, there's no sign of a Vita entry.
Final Fantasy is sort of there, sort of not, but we should find out if they have any serious intentions with the series on the platform by TGS.
Monster Hunter should be assumed 3DS exclusive unless proven otherwise.
Edit:
Before someone notes, there is a Gundam game on the system, but it's Gundam Seed, though that doesn't rule out a more encompassing title later.