Although DS is currently raping PSP, I can see how one would think that PSP has a larger audience of older gamers. Numbers and percentages yield different results. Younger gamers and non-gamers make up a good chunk of the DS user base, skewing the percentage. At originally $250, I imagine many parents said "F*ck that" to PSP. In terms of numbers, I wouldn't be surprised if DS and PSP had about the same number of older gamers (though I'm inclined to give DS the edge). So while I don't agree that PSP has more older gamers than DS, I can see how Konami could come to that conclusion.
In any case... Damn, you Nintendo fans are hard to please!
As a Castlevania fan and a DS owner, I wish this was on DS too. However, between GBA and DS, Konami released, count it, FOUR original Castlevania games. I wouldn't be surprised if Konami announced something new at E3 in July. PSP, on the other hand, is getting a remake of a 14 year old game. The ports of the original Rondo of Blood and (a newly translated and voiced) Symphony of the Night are quite attractive, but they're bonuses. It's a great bonus though the same package wouldn't fit onto a DS card.
PSP has carved a decent portion of the market. Sony has done fairly well considering how drastically every other company has failed against Nintendo. I think people take too much credit away from these companies. PSP may not be as huge as DS, but I think there's still an audience for these games or these publishers wouldn't bother. And really, DS is still getting major support from all sides.
On a semi-related note, I think another DS Castlevania is inevitable and a Wii game will likely be made too.