I think there's a simple explanation for that: in a (stake-based) system by representation disapproval has no value. Only approval does. It doesn't matter if you disagree, as long as sufficient approve...
Harsh (is it?) but true. If you disagree, your task is to alter the minds of those agreeing.
Another argument would be "brigading", an issue which we have historically seen through all voting based communities online. Brigading of witnesses and proposals would make the environment only more toxic here.
Well, there is currently no way to show disapproval so you are right. Being able to downvote proposals would fix this. Changing the minds of those who agree is Sisyphus' work.