An answer has multiple downvotes but nobody has left a comment explaining what's wrong with the answer or why they downvoted. Why hasn't anyone explained their downvote?
If someone hasn't explained their downvote, chances are there's nothing much helpful they can say, or their comment would be the kind that historically causes problems. Or, they just haven't gotten around to it yet.
We do leave comments when we can suggest an improvement, provide constructive criticism, or when we can ask for clarification. (Those are in fact the official uses of comments.) Our comment privileges page elaborates on some cases where one should or shouldn't comment. We can't always leave a comment immediately though — most of us lead busy lives, and thoughtful comments take some time to compose and write. Some of us may just settle (reasonably) for downvoting the answer, and leave the task of constructive critique to later or to someone else.
However, very often our downvoting reason is just along the lines of "this is incorrect" or "I disagree", and there's nothing to be gained from posting that. The downvote tooltip ("This answer is not useful") already expresses our sentiments. Comments with these sentiments usually cause arguments, or long discussions about who's wrong and why, and those get deleted or moved to chat. You, dear reader, may have every desire to remain calm and learn from such criticisms on your post — but others supporting or against your answer may happily engage in argument, and we don't want that.
We encourage that people who simply disagree or find an answer wrong should just downvote and move on, upvote answers they do agree with, and if they have an alternative position, post their competing position in its own answer.
RPG Stack Exchange is part of a Q&A network that tries hard to eliminate the discussion and arguments found on forums that bury the useful information, to focus on a core business of cultivating excellent questions and expert answers. We want a high signal-to-noise ratio, and we on this site take seriously the duty of keeping the "noise" portion of that ratio low. Comments are noise, and they're transient and liable to be deleted at any time unless they're doing something very valuable. (See Why are site comments being deleted?, and SevenSidedDie's additional explanation: a tidy site is safer from trolls.)
I know most of you will see this as a rant if I wrote it, so I would like to just offer some ideas and maybe you can craft a non-ranty addition to the existing answer:
RPG.SE has a community working on a very fixed set of rules. We literally have rulebooks for our hobby. And we believe due to the fact that all of us operate on the same globally valid rulebooks, we must have the same experiences.
Workplace.SE for example is very aware that a programmer in India and a programmer in the US despite both having 10 years job experience as a programmer, have very different experiences and need very different advice.
StackOverflow, while operating under the same premise of globally available rules (programming language), has a very easy option to validate an answer. Just run it through the compiler. It will work or it will not. A final independent arbiter. It even acts as an additional feedback channel. I have thought "man this is so wrong, how can a high-rep user post such BS" multiple times, only to see that my compiler could make sense of it and it actually worked. Learned something new. We are missing this feedback option here.
We are tempted to think we all must have had the same experiences, because we all operate on the same rules. At the same time we have no way to verify even the existing rules easily. I think this is one reason why we have so many downvotes without comments.