Probably because FPS do not sell well on portable consoles......
DS had a couple of FPS games, like Call of Duty, and it seems that they sold poorly (maybe one or two exceptions).
By the way, actually THERE ARE FPS´s for Nintendo 3DS, but they´re not good:
Heavy Fire
Mad Dog Mcgree
Personally, I really don´t care. There´s such variety on 3ds library that FPS´s are not missed








