Basically witness voting was inherited from BitShares, while comment voting was designed from the ground up for Steem. They're two totally different, unrelated pieces of code with two totally different histories.
It would be technically possible to add a downvote system for witnesses, but I don't know that it's something we want to spend development resources on right now when there's so much else to do.
Thanks a lot. That was a great answer and reason not to pursue it currently. Was mostly curious. Thanks again :)