Why not both
KnowYourMeme tells us this is about hard-shell vs soft tortillas:
The expression "why don't we have both?" was originally quoted by a child model in a TV commercial for the Old El Paso at-home taco kit that contains both hard-shell and soft tortillas. In the commercial, a group of Spanish speakers are seen arguing over the quality of hard and soft tacos, to which the girl proposes “Why not both?” with a shrug and puts an end to the debate. Though it is unclear when the commercial first aired, it was discussed online as early as September 2009.