A big part of the DS' software sales advantage over the PSP is just that: it was designed to be a very limited-input device. Meaning the custom software for museum tours, educational purposes, and so on all have to be made into DS carts or DS Download Station files, and sometimes even get sold on the open market. Of course, that last one leads to even more sales of DS titles, and both the first and last in that list mean even more profits for Nintendo thanks to the DS using a proprietary cartridge format.

Ultimately, storage medium and accessibility of said medium is far less important than the interface used to access it, and the overall price of the storage medium compared to competing products. That's something that Sony has a hard time grasping, as the failure of the UMD movie format has shown.

Very impressive numbers for DS and still sell more.