Simple as that.
I think that since we can only upvote comments, it leads to skew, where comments, even if they are unhelpful or inaccurate cannot be downvoted.
Given that the whole mechanics of this website is, more-or-less "people vote for things and the good things rise to the top, while the bad fall to the bottom".
Why would you use this as your model, but clearly ignore it in one place?