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The incompetent UI rehaul has meant that most users continue staying on old.reddit.com, which makes it impossible to sunset. This means that old.reddit.com works in the most familiar manner of early-2000s forums without much in its way. The incredibly late and terrible 1st party app, has meant that significantly superior 3rd party apps (without the same large scale profit motives) have gained prominence and cannot be pushed out. The ads are bad enough, that they don't have a large variety of advertisers to specifically target compatible subreddits. Imagine having simultaneously the most most engaged users and the worst ads. The censorship is amateurish, and gets beaten by a motivated bunch of idiots on a regular basis. The fear of a competitor being just around the corner (due to their own digg origin story) makes them too scared to censor beyond an unperceivable breaking point, lest they face mass exodus. Good teams are analytically user obsessed. They know their target audience very well. To be fair, worse teams ban users for what used to be the central purpose of their platform cough tumblr cough.
Its public perception is tied to the Boston marathon, pedophile-defending employees and the rise of Trump. So, there isn't sufficient adoption among 'normies'. While that's bad for monetization it slows down the rate of decline in content quality and keeps it far away enough from the public eye, that you can get away with 'better' content.
Every feature they attempt to release (Chat, live stream, video hosting) is done so badly that users refuse to bulge on previous user flows. Thus, it maintains a certain purity.
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Reddit is the most ironic victor due to the 'don't fix what's not broken' rule.
Reddit in 2015 was a pretty good website. The devs have been unable to get adoption for any new feature past 2015. Thus, they never fixed what was not broken.
Facebook frequently messes with user flows to maximize earnings. Most successful social media websites have figured out how to guide their users down an 'intended' user flow such that they can maximally profit without losing any users. Reddit doesn't know how to do either.
With that, I hope that Reddit's leaders never wisen up and start making real money. The day reddit figures out monetization, will be the day that I set forth looking for a new badly-monetized platform.