The PSP and Wii are sorta similar, from an architecture point of view, so sharing games makes sense -- even moreso than the PS2/Wii sharing does.
The PSP's main processor is actually a bit faster than the PS2's main CPU, and the coupled vector coprocessor gives it an additional boost where it needs it the most -- its probably comparable to the GameCube's CPU, from an overall performance perspective. I'd even go as far to say that its overall high-level specs (CPU+GPU+memory), matches the GameCube *very* closely, in both design and capacity.
Making a shared single-core engine, rather than trying to squeeze power out of the more complex "3-core" (I'm being kinda of free with that phrase, I know) PS2 processor and having a different engine, makes sense.
