The PSP is selling great. It's just that the DS(and iPhone) were getting more of the software experience people were shifting toward.
In the past the majority didn't want to play Final Fantasy or Pong forever, eventually that which is common between people and their habits shifts. It has been this way in gaming forever. At one point players didn't want fighters anymore or shooters.
The DS filled the gap, and aligned better with the general habitual trend than the PSP did. But by the very nature of being a portable I think the PSP is also pretty damn big. The trend is I think against the PS360 mainly, what's keeping them up there IMO is pure inertia.







