To run San Andreas on the DS, the developers would have had to:

Fans often pointed to Super Mario 64 DS as proof the console could handle GTA. However, Mario 64 is a strictly controlled linear experience disguised as an open world. It has very few moving parts—no traffic systems, no pedestrian AI packs, and no complex physics engines. San Andreas is a simulation; Mario 64 is an obstacle course. The DS was built for stylized, contained experiences, not gritty urban simulations.

For those looking for the San Andreas feel on a portable Nintendo device today: Grand Theft Auto: Chinatown Wars Review