I think it's just that it's the cool place to write blog posts now. It was similar five or six years ago with Medium, or Github Pages. I don't think conspiratorial thinking is really necessary.
Funny how quickly the worm turned on Medium due to the constant nagging and paywalling. It's looked down on in the same way as linking to a Quora article at this point.
Yeah, I don't remember the last time a Medium article trended on HN. I suspect Substack will have a similar fate eventually, for the same reasons.
I was self-hosting my blog on my own server until very recently, but I moved to Cloudflare Pages and thus far I find it to be the least-awful platform I've used for it. Cloudflare might eventually turn out to be evil, but at least since I have my own domain name and it's just a static site, it's trivial to move to a different provider or even just host again myself if necessary.
I really would like to figure out a good way to syndicate though; there's really no discoverability with the current iteration.