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 pagecomment 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.)