IMHO it's because they are two different kinds of squats. On the balls of your feet, you a ready to spring into action at any second- it is more like a quick breather than a resting position. It comes much more naturally. On your heels, you can't exactly break into an instant run.
That was just my point - it comes naturally to Westerners because they are not used to squatting. South Asians can execute either stance equally naturally.
I've seen people go from standing to squatting position (not in a toilet) and they don't go through an intermediate ball-of-foot-squatting phase.
