I really don't know what is the reason why they get rid of it but this button use to be helpful on burying the comments of the internet trolls. Facebook is returning their dislike button while Youtube is removing theirs. I guess we can't get all good things at the same time. I don't know why they removed this button. Now many trolls can easily get top comments because people can't pull them down. I guess they did it because Youtube really want people to fight.