Presumably the same reason you can't downvote immediate replies to your comments which is probably there to discourage toxic back-and-forth downvoting.
I can understand why downvoting replies is prohibited, it could harm conversations. But prohibiting downvoting in submitted stories assumes some kind of self interest, which I doubt is the norm.
I think it's generally "the submitter shouldn't be able to shape the discussion too much". I.e. also why you aren't supposed to put your own spin on a headline, text submissions are separate and not supposed to be used for links (with a few exceptions).
