The DS is a handheld. A great one, I've bought two myself, but it's comparing a handheld to a home console. I just don't find a handheld managing to sell more than a home console particularly impressive - if anything I guess I'm surprised it doesn't happen all the time.
I may just not get the demographics of handhelds right - or perhaps their growth has been slowed by smart phones, etc. this gen vs the general rise in gaming industry, but so far as I can see in theory they should sell more than consoles easily, particularly due to families. I know friends who buy one console then four handhelds because they have four kids. Really handhelds should have a huge advantage in potential userbase as they tend to be one to one for owners whereas many consoles are one to many for users.
Maybe I'm missing something.
The element that you're missing is that theorizing concerning the reason behind handheld sales trends does little to reflect the reality of it: the original Gameboy sold about 100 million units over the course of 12 years, and the GBA sold about 80 million. Both were outsold by the PS1 and PS2, and both will eventually be outsold by the Wii. The DS is the first handheld ever to sell as well as the PS2, much less better.
Whether or not it seems like this should be passe, it isn't, because it is unprecedented in the handheld space.