It works on Hacker news since the vast majority of people here share the same interests. The same is true in theory on reddit, where the voters on a single subreddit share the same interests, and importantly reddit has rules against brigading to at least try to enforce that limitation.

The basic problem is whether a downvote on a cat juggling video means "this is a bad example of juggling cats" or does it mean "I don't like cat juggling"? On YouTube, it's much more likely to mean the latter, but it's the former that's actually useful.