I don't know a lot about music composition or what's more recombinable (like how many different variations can you get out of the musical language), but I think part of it is whether the genre is tied into a broader, older artistic tradition rather than just being an occasional, immediate, one-time trend. Take the gothic / "dark" scene for example. What does Siouxsie and the Banshees have in common with Blutengel sonically? Practically nothing:
https://youtu.be/TjvvK-Rj0WI
https://youtu.be/sfN6kRAd2bQ
Different countries, 40 years apart. But they share a similar aesthetic style or "language," a similar mood or atmosphere, which also predates the music itself, and goes back to the 19th century at least, like the Victorian era. This woman's dress is made out of plastic and she listens to electronic music, but that is how the tradition as updated itself over time due to changes in technology. A lot of music these people listen to nowadays is synthpop but this "identity" never seems to die completely, it always finds some way to crawl out of the grave because it's not necessarily attached to a particular musical formula. It's an aesthetic tradition. A century ago, these people were enthralled by decaying Victorian mansions, today they are enthralled by decaying shopping malls.
Or think about heavy metal. It draws on some old artistic sources and themes that predate metal. They draw on ancient Vikings and get into mosh pits and it acts as a tradition. People incorporate metal as part of their identity.
