i honestly think that a big problem is 3rd parties release crap on the ds, and put more work into there psp releases, look at the quality and effort put into GTA on the psp. It's not a surprise it sold well.
The only way for third parties to sell well is to make good games, or at least bring established franchises. Too many companies simply make a crappy version of what nintendo has already made. How many Brain Age wannabees has Japan seen?
Final Fantasy III, Dragon Quest Monsters, even the poorly review Tomagchi title have all sold over a million. Love and Berry is getting close, and as more big franchises come to the DS you're going to have more 3rd party titles with a million+ in sales.
Again I find too often that 3rd parties are not leaders they're followers, it's like they need to be told what to sell, since Microsoft and Sony follow a comfortable formula(similiar control setup, focus on graphics) and they don't often use their 1st parties to pioneer new gameplay, it does allow 3rd parties to feel at ease developing for them, simply to use existing franchises with a push on graphics aside from other prcoccessor related elements.
But where nintendo is concerned they keep throwing curve balls: 2 screens, touch screen, motion controls, connectivity, and more often then not I believe Nintendo has this amazing vision of gameplay and they just role with it. but more often then not it takes 3rd parties a long time to "get it". Hence they often follow up Nintendo with wannabe titles, and reserve their bigger budgets and established franchises for the comfortable platforms. So in part you can blame Nintendo for not aiding 3rd parties see their vision, and you can blame 3rd parties for not taking more of a chance. now that the Wii and DS are seeing succesful hardware sales hopefully that will change.