At first it was all about presenting data in an original looking way. In the end it was about pushing political ideas in your throat using a plain bar graph. It was not about sharing something interesting you found but about taking advantage of a captive audience.
That’s how most subs end up. They have a great idea but eventually you’ve posted all the cool things. Now, you don’t really have anything cool to post that’s on topic and the sub goes downhill. Seen it happen so many times
I think another big problem is that once a sub is popular enough it will start showing up on r/all or r/popular. There were a ton of posts on r/anime_irl that were just anime memes but people upvote because “haha funny meme” and not because it actually fits the sub. They aren’t paying attention to which sub it actually came from because it just pops up in the feed and they treat it like any other social media site where upvote = I like it.