I honestly don't know much about porting or anything like that, but I will say that just about everything up to the N64 SHOULD be possible on the PSP, looking at the specs.
Since the hackers / homebrewers don't have the SDK that the game makers have, they can't just come right out with an emulator that runs at 100% speed immediately.
As best exemplified by the UO_SNES9X emulator by "y", it's a slow but steady process. Gradually, as the programmer does whatever he does, the emulator gets better and better.