I think snkrock25 explained it best.
The PSP has a lot of great games but misses the mark with the whole "portable" aspect of things (which is why it gets ragged on).
I'm sure this doesn't apply to everyone, but a lot of the good PSP games I'm interested in are ports I'd rather play on the PS2 (like Tekken, Need for Speed, WWE, GTA). Other than the fact that it's "portable" and maybe I can multiplay with other PSP users, there's no other incentive to get the same games I can play on my big screen TV.
In comparison, many of the good games for the DS are unique to the system and play differently on the DS. And they're easier to pickup for a quick game on the road.
I don't think it's fair to say the PSP doesn't have good games because it definitely does. I would say the games on a DS aren't better in quality but better for a portable system.