What motivates DS homebrew developers?
Tetattds is a polished clone of Tetris Attack/Panel de Pon for the SNES, adding online support to the puzzler. Tower Defense is an adaptation of the popular Warcraft 3 mod. The Lemmings Project jerry-rigs the PC classic to work with the DS' touchscreen and resolution limits. QWAK, a GBA puzzle platformer reminiscent of Bubble Bobble, was actually self-published with a small run of 300 carts.
So what is it that drives these homebrew developers? Several of them cite community participation or the need to fill a gap in the DS library. Others have a reason that's even more simple and obvious: They just want to make the best games they can.