If that
commandment had been included, it would've been systematically ignored by most followers and supported only by a small
minority. My justification for this
cynical viewpoint? Easy, look at the full text of the second commandment:
Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; And shewing mercy unto thousands of them that love me, and keep my commandments.
Hmm, that would rule out pretty much all non-abstract art. Now, with the wide-spread proliferation of Christianity across the world, do you notice anyone complying with this commandment? The orthodox followers of Islam do (as far as I know). So, as I said, the eleventh commandment that you propose would've been found... inconvieniant, and thus ignored or reinterpreted so that it wasn't so limiting towards the senseless slaughter of unbelievers. It would be yet another noble attempt at morality shot down by reality.