I was thinking you meant diaphragm carb's like the ones on all the 4t modern carbureted bikes & scooters, where throttle operates a butterfly valve and the slide's drawn up by vacuum. those are kinda cool i guess on 4 strokes, for smooth operators. That's what grimm is thinking of
the pump/diaphragm carbs you mean are mostly used on things which don't have a specific orientation, like model airplanes and stuff because there's no consistent direction of gravity. Its also used in karts because the g-forces when cornering shift gravity away from the vertical.
However on motorcycles/mopeds, as you turn you're leaning to counter the lateral g-force, and the gravity vector always remains parallel to the bike
So basically the carb turns with the bike, there's always gravity feed, and that works better for motorcycles.