AD shaco really only does well into squishy champs; going full AP or hybrid allows him to play around ult, w, and e without the added risk in top lane. Going AD top into a bruiser or a tank would be really difficult, he'd lose out on farm with w and always lose into champs like Darius, Renekton, etc who would easily turn a fight around and lock him down if he didn't play his range. Losing that W damage from the AP scaling would also make split pushing much more of a commitment and leaves little time for play making, which is one of shaco's strongest features. Being able to place a box and turn towards a different fight somewhere else is so much more efficient. AP is just more fun too.