Tradition. Actually, he was drawn during his lifetime and didn't complain at all. Out of respect, as his actual likeness is not known, muslims refrain from drawing him.
You could draw Muhammad if you wanted, and it would cause moderate uproar, just doing it tastefully and without deriding the historical figure.
My ,02 cents on the question.
Graven images are NOT idolatrous, as long as they're taken just as images and not as the actual thing behind it. Idols and statues are different things.


