What cause are they furthering though?

No, they gave up their nukes for the promise of Ukrainian independence and it was seen as good deal because Moscow still held sole control over all launch sequences.

It’s probably best to take this whole graph with a grain of salt. There’s already some questionable relationships in it, like for every 4th Manjaro user coming in a Gentoo user, which I find hard to believe to say the list.
Second, it’s hard to say Pop exclamation mark underscore OS is on the decline when the whole field just looks more diversified in general. Sure the hype around gaming distros from the lockdowns seems to have cooled down a bit, but there isn’t any distro that just disappeared. On the contrary, it seems to have gotten just more.
As already mentioned, we can expect another hype again when Cosmic DE launches.