Because he isn't.

but think of Pluto, mickey's dog. He is on two legs, as is Marie (aristocats)

Probably because he would need a quad-suit (a suit in which one person walks on all fours, this would be considerably more expensive, and possibly scary for young children), I've noticed that there's not very many characters at Disney that aren't on two legs for at least a small part of the film(like Rafiki), I've only been to Disneyland Paris, but actually I found that any characters were surronded by people (poor Donald...XD)
Mostly I would imagine, it's for interaction reasons, children couldn't really hug a quadroop Simba