If you do a simple Google search for "SNES9X" you'll find that it is indeed an emulator for the PC, Linux, etc. I don't see why you think this should make it better though. It makes it much easier to port over in the first place, yes, but much harder to optimize for the PSP hardware. This is why you still get slowdowns when emulating the 3 MHz system on a 333 MHz processor.
Nobody's made an SNES emulator from scratch probably because they didn't feel like it, simple as that. If a programmer doesn't want to do what he's working on, he's not going to have any interest in programming it, and it's not going to get done. SNES9X already existed, already had good support for many games, and it was easier to port it to the PSP than to attempt to reinvent the wheel by creating a new one.
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