In this sizeable feature, Edge has showed pretty conclusively that third-parties aren't releasing as much PSP software as they used to. As is Edge's wont, there are lots of graphs to make sure you know what's what, and that always brightens the mood, so we recommend you look.
Talking about PSP software in general, this does a good job of summing it up:
the drop-off is so statistically significant that there’s no way to write it off. 2008 managed only slightly more releases than the PSP’s launch year, which was only ten months long. That’s a staggering 40 percent decrease compared to an already depressed 2007.
More through there.