Perhaps also because it's naff?
The adverts are amusing, listening to Nintendo breathlessly play up the console's "amazing graphics", that still aren't on a par with the original PSP., and the "advanced connectivity" that basically equates to "online multiplayer." In fact, the current ads barely mention 3D at all, to be honest.
There have been a good number of DS revisions now, and I've owned several of them. In truth, I'd rather have a PSP Slim than any of them, hands down, no contest. The DS range focuses on gimmicks and "family friendly" shovelware trash, whereas the PSP has titles that look suspiciously like actual games.