There are ways to leave a comment while reverting a post, but they're not obvious.
Please make it more obvious how to comment while reverting.
Clicking rollback should definitely have an rollback-reason or comment input, same as editing does.
I guess a slightly more drastic solution would be to remove the rollback button, and rename the "edit" button on the revisions page to "revert to this revision". Not only would it inherently prompt for a reason when "rolling back", it better describes what the edit button does. Plus, it gives a nice preview of what the rolled-back post will look like.