>I always wonder why they aren't as popular now.
IMO it's a lot of reasons.
A big one, that was the case even before social media, is the inherent geekiness of it. "RSS" is an acronym after a geek's heart, most people don't like it.
The subscription process is complicated. Go to the site, find the feed (which one? There might be a bunch! eg, main feed, topic feed, comments-per-post feed, etc), take that URL and paste it into your feed reader, then look at that and... you got linked back to the site you were on? Yay... There have been various attempts to streamline this via in-page "add to reader" widgets back in the day, browser plugins, etc. But it was never as frictionless as the Facebook/Twitter/Reddit "follow" button.
Responding to things is harder. There's no comment/like/whatever button right there, you have to go to the site, figure out what's going on there, maybe sign up for yet another damn account if you want to comment. Some sites don't even have comment sections! Or maybe they point you to FB or Twitter anyway. Again there were partial solutions back in the day, but nothing as easy as modern social media.
Last for today, it's not nearly as addictive. Some people (including the author and myself) like that about it... but it doesn't drive adoption nearly as well.
>I assume they'll be popular again some day like vinyl and static web sites.
I think they'll always be niche-popular with geeks, long-form content creators, open web advocates (the "EFF crowd" I guess) and so forth. But I don't think it'll ever be mainstream. At best it could serve as the substrate for something else that becomes mainstream.
Anyway, just my 2 cents from hanging around this space for a decade+.