Because it is good for a very narrow scope of things, once you go beyond that, it becomes a kludge to work with.
People hate on React but it is the product of 2 decades of various experiments. It is popular because it really works.
Been building a fairly medium to large sized web app in HTMX these last few months and if done correctly, it can be done fairly cleanly and it's best combined with templates of some kind (e.g. the templating in Golang or in Django).
“If done correctly” applies to about everything though.
Problem is that a lot of people don't want to spend some minimum effort getting some fundamental understanding about the technology they are using. They just want to rush it to build something. It's been like that forever.
I see you're a true Go programmer. "You're holding it wrong".