Reason #1:
GBA. Consumers aren't purchasing the DS en-masse since the GBA is still selling very well. Even @ 80k/mo, thats FAR better than Japan, which dropped the GBA like a smelly turd.
Reason #2:
PSP. The PSP still sells very well in the US, and unlike Japan's 5:1 ratio, it's more of like 2.5:1 in the US. There are only so many handheld sales to go around in the US.
Finally, to sum it up: The US handheld market sells around 700,000 units a month. In Japan, it's around 600k. However, the ratios are far different. Not only this, major uber-J games like Brain Training, and such have not caught on in the US like Japan. They sell well, and Nintendogs is huge, but there aren't any 4m+ sellers in the US aside from Nintendogs and Pokemon (shortly). In Japan, 1m is nothing anymore. In the US, it's still kinda sacred.
Back from the dead, I'm afraid.
