Dropping the GBA slot lowers production costs; aslo since GBA are no longer produced (are they) Nintendo does not get a license fee. Also they can now shovel GBA games into WiiWare to sell to all the new gamers on their idiot box (Wii), thus getting their fees back.
Actually doesn't bother me too much. I am not a big fan of digital distribution of new games (STEAM for example), but I don't mind older smaller games being downloadable...