The PPC is more than capable of running Genesis, SNES, PSX, Amiga, and plenty others at full speed. It's possible.
But in some cases like Genesis and Amiga, it just hasn't been done. That DOES NOT mean it CAN'T be done, it just HASN'T. Genesis (PicoDrive, MorphGear module) has been long since abandoned and not picked up by anyone else, so it's still old, buggy, incomplete and unoptimized code. Who knows, maybe someone will pick one of these emulators up one day and finish it off.
The Amiga, as another example (PocketUAE) is nearly feature-complete but just needs some hardcore optimization.. maybe a bit of assembly in there - which I *THINK* and *HOPE* is in the pipeline. It had a nice new update recently - let's just hope it doesn't get abandoned too...
The only reason why PSP might have more full speed emulators is that it is a relatively cheap device compared to the PPC, and one hell of a lot more popular. It has a wider audience, both to users and developers alike.
It's just a shame that the Pocket PC gets less attention, because in terms of its sheer versatility and its wide range of features in general, it's a superior device.