I think there are a couple of things here.

First, RSS has a bit more friction. Smashing the follow button on Twitter et al is faster than adding the feed to your RSS reader of choice unless your OS has support for default RSS app.

Second, discoverability. Just like with any distributed system vs monolithic platform, you need to find what to read yourself. For some niches this works well. If you are a software developer/hacker, you are more familiar with blogs in your area of interest. But if you have a wide range of interests you’d need to find the blogs yourself and hope their RSS feed is well formatted.

Third, the algorithm. A monolithic platform can do more to try to mix in new content based on your interests and intelligently mix up the content from sources you follow. This is of course controversial because feed algorithms can also try to cram bullshit into your feed or hide important stuff from you or create an echo chamber. But in the best case scenario they can also expose you to new sources of content you wouldn’t have found otherwise. An RSS reader would mean it is up to you to do this discovery which is more friction.

And ultimately content creators realized that they get more eyeballs on their stuff by using platforms like Facebook, Medium, Instagram, Twitter, than on blogs especially since blogs tend to be then repackaged by blog spam bots, Google’s AMP, and now LLMs.

So IMO RSS is just too manual and requires too much work. And of course since you can’t effectively advertise through it there is less incentive for creators and platforms to support it.