The two languages have nothing in common beyond both being functional, and it kinda feels like someone posted a "Elixir and Elm" blog post in the early days and it all just snowballed arbitrarily from there.
I really like Elixir, a lot. I’ve had Erlang experience and I’ve never understood the hate around it’s syntax. Elixir is definitely more friendly in that regard and I’m glad it’s bringing the tremendous value of the BEAM and OTP to more mainstream development circles.
I think that Elm and Elixir fit their respective development niches (UI, distributed systems) extremely well, both have similar community cultures, and both started gaining some traction at around the same time.
Elm on the other hand inherits Javascript as its foundation, and the browser as its target. Javascript has an anemic standard library, and isn't a functional strongly typed language to begin with. Now since the browser is the target, file size is important, preventing Elm from simply shipping its own runtime + standard library.
That's good enough for most people. From experience switching quickly between the two paradigms takes a bit longer than staying with the same one. That is once think about reducers, closures, immutable data it's easier to stay in that mindset then going back to mutable objects and attributes.
I love useful constraints.
Though also Python in a sense already has some extra state visibility by passing self around in member function as opposed to hiding in an implicit this.
I was excited when I discovered accidentally that tuple unpacking in function heads was supported in Python, then depressed when I discovered that was dropped in Python 3, "because no one uses it". Sigh.
If you need raw performance, the JVM is definitely better. If you don't want to have to deal with the inherent tension between a FP language and mutable surroundings, the BEAM is worth a look.
It has the drawback of the frontend language not being as tiny as Elm is, but it has the benefit (which I think is no little benefit) of having the same statically typed language (F#) on both front and back end.
Whether it's "Relatively popular" is hard to say. It's tiny as far as web stacks popularity goes, but in completely functional web stacks it probably has taken a bite already.
Or perhaps, some day, https://github.com/clojerl/clojerl.
[1] http://lfe.io/