Easier to eat in hand. Those big triangles of round pizza flop all over the place, and you have to fold it over to eat it without getting hit by food shrapnel. Also, if you're feeding children, little squares make it easier for little hands to hold, and less is wasted, because they're eating smaller pieces, so they don't take a few bites of a big piece and leave the rest. Bigger folks can just eat more little pieces. ;o)
Makes good sense. To me though, a piece of pizza without crust is wrong on many levels.