We can cast up or down votes on both questions and answers. But, as far as comments are concerned, we can only up-vote them (or flag them if necessary).
Now, just like we have some comments that are up votable, there are many comments as well which are down votable.
Q- Why is this feature disabled? Are there any specific reasons?
As suggested by @TheDestroyer, this is already covered in below post & its duplicate:
Why was "downvoting comments" declined? Should this feature be revisited?
To me the major blocker to avoid this feature is "edit" option. When a Qn/Answer is downvoted, it can be improved by other users with "edit" option, at any point of time.
For comments, this facility is available only to diamond Mods (i.e. they can edit anyone's comment at any time).
Since normal users can't edit their own comments after 5 minutes, it's little unfair to allow downvoting.
