Could it just be an organizational problem? When you start using HTMX in your company your neat distinction between frontend and backend devs collapses and everyone suddenly must be a full-stack developer. When you can avoid that additional cost by simply continuing to use React (or others), why wouldn't you?
Edit: As a side note, I have never used any large JS framework, only HTMX. Just trying to put myself in the shoes of happy React users and such.