A lot of forums also have tyrannical mods, but the key difference is that a forum doesn't have the same unique authority that a subreddit with the best name for the topic has. If a forum has bad mods, other forums are on a bit more equal footing as competitors. The discussion thread format is also less prone to bad moderation than reddit's link/image focused post threads are, and usually also forces the mods to actually participate in the community. Plus if someone breaks a forum rule other than one related to creating threads, other users can come scold them for it more easily (necroing is a common example).

Reddit isn't built for community discussion and is getting worse at it as the admins try to turn it into instagram.