1. People learn more than 1 language. What I'm referring to here is complete beginners who need to learn -how- to program. Once they grok that, it's not difficult to move to other languages, obviously with some exceptions.
2. It's not the job of a -language- to teach everything about programming or networking or computers or various protocols. That's the job of a curriculum.
3. There are way more $major_lang jobs than jobs for D, Clojure, Lisp, Crystal, or any of the other languages that are significantly less used. The sky is the limit for how many languages people want to play around with and learn, once they know enough to start learning other languages by themselves. No recruiter is gonna be like "oh, you learned $major_lang... Even though you likely applied to this role because our stack involves $major_lang and that's a language that you know well..."
4. Help, resources, meetups etc for $major_lang are orders of magnitude more plentiful than for D, etc. Imagine telling a student "no, no, you need to prove yourself by making forum posts for <niche language that I personally like and use> instead of having stackoverflow questions or tutorials to follow" in the case of getting stuck. Of course students should strive to do as much as they can themselves, but you would be surprised to see what beginners get stuck on and just need a gentle push to get going again.
5. Being able to be independent is more engaging than having to constantly pester a teacher or a forum. If a student wants to be a go-getter with e.g. python or lua, there are communities and resources to build games or websites, etc. And those already have had common stumbling block questions answered because they have large communities around them.
For the goal of teaching someone how to program, any language other than the major ones like python, javascript, ruby, c++ strictly make it more difficult for students because of the smaller ecosystem, lesser amount of documentation, and smaller community in general. I have nothing against D or other niche languages, but I'm against picking languages just because I'm familiar with them.
It can be difficult for experienced programmers & power users to remember that there's a sheer amount of information that needs to be learned in. There is no reason to make that more difficult for students.