Nobody respects etiquette anymore, and rather than upvoting valuable content they upvote whatever they like. As an extreme example, you can make the best, most honest, rational argumentation of a political issue, if the users don't agree with your stance it's going to be downvotted and hidden. Similarly, they will tolerate content that might be against the rules or the spirit of the subreddit if it's something they agree with.
On top of that, the vast majority of the time moderators enforce echo chambers themselves through bias, with a few of them going as far as banning every single user that posts in communities they disagree with, even if they have never engaged with the community they themselves moderate.