I think the content on newer platforms (TikTok, Reddit, Twitter whatever) is presented in a more mentally stimulating way, so people just tend to gravitate towards those things. And yes, ads too probably.
That being said, any time forums come up in discussion there is so much nostalgia there. Messing around with forums software like vBulletin and stuff as a kid taught me a lot about the internet, ftp, php, hosting things, graphics design, etc.
I wonder if older people feel the same way about BBS?