This is actually one of my favorite subjects, and a host of factors are involved, far too many to elaborate on here (indeed, if one had the time and the wherewithal, a thirty page analysis paper could be written on the subject).
Aside from the reasons already posited--the shortcomings of DS hardware in comparison to the Playstation, the apparent lack of artistic vision at Konami currently in regards to the franchise, and the wonderful effects of rosy nostalgic hindsight--one other facet I've been turning over lately in my head is the seeming dynamic of DS development itself.
With the DS, and, to a similar effect, the Wii, there seems to be a mentality amongst software development on the part of third party companies that "good enough" is more than enough. The DS and the Wii, of course, are practically printing money for Nintendo and everyone involved. The absolute wealth of disposable party, mini-game, and casual gamer oriented games on both systems, and their near-inexplicable phenomenal success, is evidence of a software market where insubstantial obligation is placed on the developer to actually make a good game.
In a market where "Catz," "Dogz," "Horsez", "Aquarium," and "Mario and Sonic at the Olympic Games" are nearly tentpole marquee attractions for the DS, what incentive is there to really invest the necessary time and effort to actually write and produce a really good game? I'm not saying that the Castlevania efforts on the system have been of the same caliber as the "Petz" riffraff mind you, and I graciously acknowledge that there have been some pretty well done third party games on the DS, but overall, the necessity isn't there to devote the hard work needed to push the system to its limits as should be done, and that, I feel reflects in the unimpressive entries we've seen on the platform in the Castlevania franchise.