@Felthry AFAIK, it's because together with the 7:6 pixel aspect ratio it has, it neatly projects onto a 4:3 screen.
As for why they went with the non-square pixels to begin with, prooooobably a design quirk left over from the NES days (16:15 output resolution aspect ratio); as for where that came from, the best guess I have is making the best out of hardware limitations.
@Felthry From a technical standpoint, yes. From a design point of view, they may just as easily have gone "fuck it, designers on our previous platform already rolled with this without complaining -too- much, it's part of our brand now" since there really wasn't anything stopping them from just sticking to 4:3 this time around.
Also lack of detail in the horizontal dimension is harder to spot than lack of detail in the vertical one, so there's that.