Comment by fundmondawyaya

I don't have any primary sources, but in a nutshell, the problem was ROM sites.

You could download every NDS game ever made, in every region, in a few days.

Small indie ROMs might be 2-16MiB, but the big Pokemon/etc games went up to 128MiB. With 8-64GiB microSD cards, you could fit a library of games into one cartridge.

Personally, I think that most of the people who pirated NDS games would never have paid for them, so there may not have been many lost sales. But I also can't deny that the small game developers got royally screwed, and IMO the 3DS may have suffered from a dearth of creative small devs.