Part of it was the platform getting worse, sure. All the reasons others said, the algorithm getting worse, the API, etc.
A big part of it is the people. The more people from Facebook and Tumblr and other big apps came to reddit the worse it got. It also happened at a time that subreddits and the algorithms and rules were creating stronger echo chambers. What it created was incredibly toxic braindead circlejerks.
I'm so tired of the braindead circlejerks. Basically everything was. I'm so tired of seeing copy pasted musk threads where everyone tells everyone else how much they hate musk saying the same lines over and over, for example. Or repeating the same misinformation about topics they're fired up about because it fits their bias, while also complaining about misinformation. Pushing the dawn project videos for Tesla circlejerks or literally anything related to crypto for example.
I hope one of these new forums or social media sites popping up creates good communities and somehow avoids creating these echo chambers and reinforcing them with their algorithms, moderation, general design, etc.
I miss old reddit and old forums and IRC so much these days. They weren't perfect but they weren't as bad as modern social media.