If you think that a technology/language/etc is cool and better than the rest but it is not as common as <mainstream thing>, of course you’d be excited to learn that a company is using it.
If you’re a company that chose a very niche language because <a number of reasons> and discard an applicant because he likes the same very reasons, LOL, you’re a bunch of idiots
Well, they're moving away from said language, so why would they want to hire someone who joined because they liked Elm but then doesn't get to use Elm at work? They're more likely to leave the company than someone who doesn't really care so particularly about Elm (or whatever language or tool it may be). So in OP's case, it's a sound business decision.
Adversarial take: if your company says it does Elm, then hires people (who took Elm as part of the decision of joining) and then decides to stop using Elm, why should we push the guilt of leaving to the employee? I mean, it looks a bit like bait-and-switch, and "We the company altered the deal, pray we don't alter it any further".
How is that employees should be loyal to any decision upper management does? What's the limit? What if the company decides to go into JBoss/JQuery? What if the company decides to stop doing software altogether? What if the company becomes a mercenary army a-la-Wagner? Where do we place the line between employee is guilty for not continuing/company is guilty for changing everything that matters?
I think you're reading guilt where there is none. I just mentioned that it's a sound business decision, as well as a better decision for the employee who likes Elm, to not hire people who like using a tool your company no longer uses. There's no guilt on either side, it's just pragmatism.
If you’re a company that chose a very niche language because <a number of reasons> and discard an applicant because he likes the same very reasons, LOL, you’re a bunch of idiots