Hmm, because we want to?
And Tolkien, when he was aware of an error in already published text, sometimes tried to work with it in some internal way. Not always, but sometimes he would, in my opinon to help with the inner consistency of the subcreated world.
In this case we all seem to agree that Tolkien just forgot this line, so why not try to make it work (I say).
No Tolkien wasn't perfect, but at times he played this 'game' himself, no?