It is usually niche languages, which are ahead of their time and coming up with interesting ideas or get rid of annoyances of entrenched languages. I want to be free of the annoyances of the languages, which I use on the job. I want elegance in expression and to be able to do things very cleanly.
There is also the aspect, that many only learn some mainstream language and do not look further. This in turn limits what is used in many workplaces. It is simply people not further exploring and educating themselves. I do not say, that they have to do that in their free time, but I myself, in my free time, do not want to be limited by mainstream choices, like I would be in most places at work.
I also need new interesting things in my software life. The ordinary stuff bores me. I try to learn more things. There are many things to learn from niche or at least non-mainstream languages.