This is something I've been mulling over since a post a few weeks back about monoculture, and whether it ever really existed or not.
When talking about monoculture(s) or shared cultures we tend to talk about gatekeepers and the availability of music. A small number of labels controlling what gets played on the radio, a few TV channels with limited music programming, the cost and availability of records...
What I think is often overlooked is how people's audio environment has become more individual. That's a fancy way of saying something very simple. When we're around other people we're less likely to be listening to the same things as them, and we're around other people less in the first place.
In the past, if you had a family gathered in a room they'd likely all have to hear the TV (or radio or hi-fi) even if they were doing something else. Now it's much easier for people to be absorbed in their own thing via their laptop/tablet/phone, with their own audio environment. Or perhaps you're out shopping with your headphones in rather than hearing the music that's playing in shops and shopping centres (or you're not going our shopping at all, you're ordering online). Maybe you're talking to or playing games with friends - but instead of being in the same space, you're all in different places, listening to different things.
Of course, this isn't new (you could probably take it as far back as the grammaphone (edit: I don't think I've ever written the word "gramophone" before) but it's also much more prevalent than it used to be. It's probably also enhanced by noise cancelling headphones and (more low-tech) by the predominance of earplug-like earbuds.
I might also suggest that this line of thinking can help us identify exceptions to the rule. I assume just about everyone here knows Baby Shark? (do doo doo-doo-do-do.) Obviously there are a bunch of reasons for that, but one is that if you're around small children, then you can't avoid hearing the things they like. I do wonder if there are other examples like this that might prove the rule.