You're comparing the market as a whole to the Castlevania fanbase. The vast majority of sales come from fans of the series itself. That's not really the problem here. Konami's half-assed attempts to appeal outside of the Castlevania fanbase haven't changed the fact that they're preaching to the choir. Castlevania games sell to Castlevania fans first and foremost. Konami is guaranteed that many sales so long as they appeal, and in some cases, adhere, to that audience. They can try to create this or that spinoff but that inevitably splits the audience and if it sucks, it splits the audience even more.
Well, they only have so many teams that are capable of making a good 2D Cv (I'd guess ONE) so throwing one on the home consoles means one less for the handhelds.
There are 3 handheld Castlevania games on DS vs none on consoles. And it's ridiculous to think that only one team can make a good 2D Castlevania game. Replace "2D Castlevania" with another series and try to make the same argument. Circle of the Moon was made by a completely different team than the one that made Symphony of the Night. While that team no longer exists, it's proof enough that there are teams capable of making good 2D Castlevania games, so long as they give a damn about quality.
And "way more expensive?" Please. Consider a game like Odin Sphere was made by less than 15 people. 2D games don't need nearly as many people or budgets to develop.
Also, I don't think motion controls are an issue here. First, it depends on how they're used. They worked fine in Shake It. Second, who said they even need to use motion controls? There's the classic controller and the GCN controller. And before you point out that extra accessories are needed, consider that if the game calls for it and if the audience is passionate enough about the product, a $20 ($10-15 used) classic controller isn't too much to ask, especially since it can be used for other games. Think of how many people paid $250 specifically for Wii Sports. While asking consumers to pay for something extra is fairly risky, people will spend the money if they deem it money well spent.