This forum is giving me a lot of trouble, so apologies for the above post, I can't seem to edit it.
In any case, the point I made in the post I wrote was this:
The DS was effectively a successor to the GBA. It was backwards compatible and played host to a variety of GBA franchises. GBA owners had a lot of incentive to adopt.
The DS launched $100 cheaper then 3DS did, with a Mario game, in the middle of the holiday shopping season. 3DS launched in the middle of March with Pilotwings. At this point in time in the DS's life span, it had already seen it's second holiday shopping season, got its first two evergreen titles, and was beginning to take off.
So all things considered, 3DS is doing pretty well. Let's also remember that the DS is still doing pretty well. It's effectively siphoning sales from the 3DS. Consoles typically do better once their predecessor is dead.