Well, for Pokemon B&W2, they already explained that. People who played the first B&W already have a DS, so making the sequel on the same system would result in the most sales.
I honestly don't think Nintendo is continuing to make DS games because the 3DS isn't selling well (which if I recall isn't the case anymore), I think it's because the games that they are making for the DS wouldn't make use of the 3D or the tech that the 3DS has to offer. Lets face it, if they made a basic DS game for the 3DS, plenty of people would complain, and they know it.
Theres also the fact that, when a new system comes out, they do tend to make games for the old one for a while after. I mean, when did they finally stop making PS2 games after the PS3?
I honestly don't think Nintendo is continuing to make DS games because the 3DS isn't selling well (which if I recall isn't the case anymore), I think it's because the games that they are making for the DS wouldn't make use of the 3D or the tech that the 3DS has to offer. Lets face it, if they made a basic DS game for the 3DS, plenty of people would complain, and they know it.
Theres also the fact that, when a new system comes out, they do tend to make games for the old one for a while after. I mean, when did they finally stop making PS2 games after the PS3?