I think it was a logical move on Nintendo's part. Sales would get much higher if they released it in the DS, and a game for it is compatible with both systems anyway so. Since it's still Generation V and they sort of it to be more traditionally connected to the other DS games, they went with the same handheld. The DS isn't dead just yet, I guess.
I'm personally happy they made this decision. B/W was already a DS game of great quality, and not jumping into foreign ground would maintain that same glorious quality that I enjoyed. This is running on the assumption that if they did go with the 3DS, it would have been too soon at this time, and the quality probably wouldn't be too much of a step-up from the recent past. That would be disappointing. If they do make the R/S remakes, they should go straight ahead with the 3DS, haha. But B2/W2 belongs with DS compatibility.