Orcs were corrupted/mutated elves and uruk-hai were corrupted/mutated humans iirc but could be mistaken
It wouldn't necessarily be cannibilism then.
I think I heard it was because of the Christian roots of his LotR writings: are orcs fully evil and impossible to redeem (which would go against Christian belief on salvation and some of the themes of the LotR) or could they be redeemed, which would be against how they are portrayed in the books.