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) less than a week.

React is so simple to use that you definitely don't need to hire a "react dev" to "handle all the complexity". Like >95% of the effort is just understanding the basics of standard front end technologies (HTML, JS, and CSS), something that "back-end" devs (especially ones that label themselves as such) are by no means guaranteed to understand, which is an issue, since you'll have to understand these things even when working with htmx.

My feeling as an old-time "web developer" who has been bullied into becoming a "back-end dev" is: thank you, I think I know quite well the basics of standard front-end technologies, but suddenly some people started yelling at me "you're a grandpa, now you can't send HTML from the server anymore, it's lame, you have to send to the browser a JS app that will manipulate the DOM live instead". This new (in 2015) approach has flooded our brains with a deluge of libs, frameworks, tools, concepts, problems, etc. which are a lot more than 5% of the effort of bootstrapping a React app, let alone optimizing it and maintaining it. I know people who's main job is creating React apps and who are overwhelmed with the complexity of the stack. If you don't hire someone dedicated, the rest of your product (domain rules, database optimization, infrastructure, devops, etc.) will suffer.

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.

Don't underestimate the learning commitment it takes to learn react. Learning the basics of react and its component architecture is one thing, learning all the tricks and gotchas related to hooks, accidental re-renders, accidental no-renders, etc takes time.

Throw in the usual pile of libraries used with any larger react app and it can easily take months to really get moving.

Everything has tricks and gotchas, even htmx.

I know so-called "senior frontend engineers" whose speciality is React, and who can't handle React. So all others...

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.

It reduces development complexity.

- No complex build set-up (just link the script & use it)

- Less congitive load when reading the code. Especially if it's not yours.

- Very decent learning curve.

But all it is is fancier fetch()s. It can't remotely approach the functionality of React, so it just shifts any complexity to different places.

Yes, it shifts complexity to the backend. Which will already have it anyway (because it has to), has better tools to manage it, you can use literally anything you want instead of being forced to use javascript, has (relatively) instant access to the authoritative source of truth, and isn't resource constrained (at least not the same way that the client is).

so "just shifts it somewhere else" might be underselling it a little

And waiting much longer than an instant for a million server requests every time a template changes on a page is not a good use of those resources.