Earlier quoted context omitted.
Cost. React will require you to hire a React dev to handle all the complexity. htmx will be mastered by your "back-end" devs (who actually are web devs) in less than a week.
What kind of web dev can't handle React? Meanwhile, htmx uses clunky, non-standard attributes that rely on logic and templates that are split up in a million different places. Plus it requires a context switch to do anything client side.
This thing (and others like Vue, let me be clear) adds many layers of complexity over the web platform (client-side routing, data fetching, state management, rendering, etc.) and many third-party JS libs that you have to constantly update. You just can't say that it's standard/base web development.