I can't comment on the Elixir stuff since I haven't really touched it at all yet (only ever seen one job opening in hundreds over 5+ years mentioning Elixir - only one, total!). But I have a theory on the rest...

I think the actual reason why it's not more popular is something I personally call "The Tyranny of the Masses". In short, what's popular/flavor-of-the-month is automatically "right", and everything else is automatically "wrong" or "high risk" in our industry. These are of course not real demonstrable facts, but people's perceptions that influence choice of technology stack.

Take JavaScript/Node.js for server-side apps for example. MASSIVE ecosystem. Near total ubiquitous skill availability in the developer market (with one major caveat I'll explain below). Not perceived as a rare or difficult skill to pick up, so salaries can be on the lower side than, say, a C developer with Linux kernel experience, for example. To employers and managers, this is a much more attractive mix than something where all of the above are in any way seen as "less true".

Compare that to Elixir. Not around nearly as long as JS, far fewer learning resources, relies on perceived turbo-nerd/esoteric "Erlang" runtime, so support availability may be harder to find and more expensive (so what's that mean for the employer's support and availability contracts, uptime agreements, and/or regulatory environment should something go sideways?). Not nearly as many available published libraries as on NPM (yes I'm sure Elixir's are far higher quality, but remember: it's perception, not fact, at play here). Skilled hires in market not nearly as common or ubiquitous as JavaScript devs are. Nobody has 20 years of Elixir experience. 20+ JavaScript experience does exist though (again, perception matters, not the fact that JS 20 years ago was laughably bad and would almost never even run today!).

Then there's the "safety" factor for production apps. JavaScript? Billions of apps all over the place. Nearly ever prod scenario imaginable has happened and 90% of them have at least been blogged about. Async/await in single threaded runtime using multiple processes for concurrency? Yep, somebody's done it. True multi-threaded single-process concurrency? Yep. Big, perceived as financially stable tech company behind it? Yep. Large market/ecosystem of firms that provide high quality support at reasonable prices to compete for business? Yep. Elixir? Arguably none of that, and definitely not to the quantity - again, not quality, quick-glance perception, optics here is all people look at - that JavaScript has behind it.

Is Elixir a better language, runtime, and ecosystem than JavaScript/Node.js? Almost certainly, and you can probably prove that pretty objectively with only some very obscure/esoteric exceptions having merit. I mean, dude, it's JavaScript, that's a low fucking bar! (Yes I'm a JS hater, go ahead, flame away! "YOUR BOOS MEAN NOTHING TO ME, I'VE SEEN WHAT MAKES YOU CHEER!" - Rick Sanchez)

But is it popular? Not compared to JS! JS is like that air-headed valley girl high school cheerleader that every guy wants to sleep with, and Elixir is that nerdy girl with a good heart, a great head on her shoulders, who can respect people and while she covers nearly every inch of her skin with clothing, leaving everything to the imagination, even a blurred photo would tell you that underneath all that, hot damn! She's the total package. But you want the cheerleader because of the popularity contest. You'll be the big man on campus if you get with her! Well, maybe not "the", more like "a", one in an ever-growing series.

Fast forward to 5-10 years post HS graduation. Cheerleader? Trailer trash with 6 kids on food stamps and zero drive in life whatsoever. Nerd girl? CEO of her own startup, and somehow even hotter now than she was back then!

Oh how foolish we are for falling into the trap of popularity. And while some may feel justified in their subservience to that popularity because reasons, some of them even good, we'll all feel curse the lack of choice, the absence of competition, the void of creativity and innovation - that we inevitably acquiesce to the clamors of the mob. Our future's bright hope will be made dim indeed when we succumb to the Tyranny of the Masses.

EDIT: The caveat on JS skills I mentioned above? Almost everybody who says they "know" JS is really at best an intermediate-level developer with it. It has a lot of special use cases, variations in how things work depending on engine, execution context (server app? browser engine?), and even today in 2021 we still see even self-described "experts" telling you to use alert() to get debug data instead of at the very least console.log/warn/error, etc. The language has made some frankly just plain weird (IMO flat out bad) design choices, like allowing more arguments to a method than its signature declares, and used to be so vastly snowflaked (nearly zero consistency between browsers) that you absolutely had to write an ENTIRE new JS app for each browser you wanted to target. Granted, that was the Netscape Navigator days, when Internet Explorer (Exploder?) was version 5, the rightly-maligned version 6 didn't exist yet, Firefox was a twinkle in somebody's eye, and Microsoft even shipped IE for the Mac (like OS X 10.1 or 10.2! Remember Aqua, the striped textures? Ahh, the good 'ol days...) It's come a long way since, but it still fundamentally suffers from having more bad documentation than good, and while most consider it a "lowest common denominator", that's unfair to people who really are true experts in the language, who will tell you that when you get real deep with Node or V8, you can see some gnarly shit. It's a god-awful security nightmare; why the hell would I let whatever website in the world run whatever code they want directly on my machine all willy-nilly, automatically accepting every API call by default, denying almost nothing, with unrestricted access to some pretty advanced/low-level APIs that can do some serious damage if abused? And people say Flash was bad (well, they did only after Steve Jobs made a big deal about it; he wasn't wrong, but virtually NO ONE hating on Flash back then had any idea how it even worked before Steve bitch slapped it into oblivion and rightly so). Its syntax is arguably archaic (we don't need semicolons to end statements/lines these days, language parsers are much more capable now; look at Python, Ruby!), it's littered with inconsistencies in the language itself, and since everybody's a hammer (JS "expert"), everything looks like a nail (a suitable use for Node/JS/DOM/Script), even when it's damn well obvious, painfully so, that it's absolutely NOT. All this, and yet everybody's somehow an "expert" with JavaScript. Really? Really?