Failure? The PSP has sold 70 million units. That's a lot. It's the only non-Nintendo handheld that got a proper sucessor as well as opposed to slowly dying out. It may be far behind DS numbers, but taken in the context of previous non-Nintendo handhelds and video game systems in general, it's doing pretty okay.
The hardware might have sold well but the games didn't due to the fact that the PSP was rift with piracy. Also the UMD format failed to take off.