I don't know, they started putting in some real effort I guess (compared to the first two years of the system's life-cycle) now that they know exactly what sells on it, and what demographic they should cater. It's like the PSP has finally found an identity.
There were plenty of games in the first two years of the psp that had a lot of effort put into them, though. Like Lumines, Wipeout, Ridge Racer, Ultimate Ghouls and Ghosts, Megaman Maverick Hunter X, Mega Man Powered Up;etc