The description for the reversal badge tells me:
Provide an answer of +20 score to a question of -5 score. This badge can be awarded multiple times.
Now, most questions with a score of -5 or below have that score for a very valid reason (on non-meta sites), they are of low quality, show no effort, or is a broad and/or opinionated question. A good majority of these downvoted questions are also good candidates for closure. As such, many of these questions get closed, preventing people from answering them. Even if you manage to stumble across the question before it got closed, adding an answer would only result in you getting downvotes for encouraging answers to a poorly thought question.
Another possible criteria could be popularity. Questions that recieve lots of downvotes are hidden from the front page, meaning that it will recieve fewer views, and less chance of your answer being seen.
It takes an exceptional skill to provide a useful answer to a question that attracts non-useful answers. It also takes incredible luck to have enough eyes on a low quality post to actually get 20 upvotes (assuming you dont get downvoted at all) on such a post.
This situation is, of course, different on Meta as votes also indicate disagreement along with the reasons on other sites. An idea can be unpopular, but still worthy of a great answer to explain why such said idea is a bad idea. That's why getting the Reversal badge would be considerably easier on Meta.