[Formendacil]

I think it's partly a case that, as Tolkien's metaphysics around Elves got fancier and fancier, he became more and more leary of trying to explain Orks as a tortured derivative of them. Look at all the charts on ageing and generational growth in The Nature of Middle-earth and I think that if you tried applying it in any form to the Orks, it'd be difficult--especially in light of all the talk of "orks multiplying" in timeframes much closer to Mannish generations.

Of course, Tolkien could have explained differences in things like this as a result of Morgoth's tampering--but I suspect he didn't want to give Morgoth more credit in being able to tamper with the fundamentals of Elvish nature than necessary. (Same with Men too, but the most of the difference between Orks and Men can be put into degrees and gradations--as he liked to remark, there are Orks enough among us. And perhaps--having used that metaphor all through World War II and beyond--he felt that helped explain the truth of the matter.)