When downvoting on meta I get a "please consider leaving a comment on how this can be improved" message.
In meta votes mean different things, so I think this isn't relevant. Can it be removed?
Another reason for this change could be that most people on meta know that (when to comment) already.
Note that this might only appear for users with lower rep.
See my comments on kamui's answer for further explanation as to why I think comments aren't needed in the majority of cases.