It isn't only the decay. People change. Who you were online at age 23 and 30 and almost definitely 39 is bound to be vastly different as your priorities and real life relationships change (like marriages, etc).

And the topics changed faster. People into mainframe OSes had the same conversational fluidity in that for decades. Leave linux for too long and everything sounds like vocabulary from an alien world, now. And so many 'technologies' with it. True probably since cloud and containerization. So people have less in common technically in those communities and as more career branching happens, people get nichier. More interest bubbles. More and more people in core areas, making it hard to not be overwhelmed simultaneously.

In the late 80s I was working at Rabbit Software in Malvern PA (they made IBM 3270 terminal emulator software). I used to car pool to work from Phila. with a brilliant woman who was a lot older than me, and had a solid lifetime of IBM mainframe experience. I was a Unix hacker with just a few years under my belt.

We realized very quickly that if there was one thing we couldn't talk to each other about, it was computers.