I was thinking about my academic experience with Smalltalk (well, Squeak) a while ago and whether I would like to use it for something, and it got me thinking: sure, it's as good and capable as any popular language, and it has some nice ideas, but there are certain languages that are already well entrenched in certain niches of programming (C is for systems programming, Java is for portability, and so on...), and Smalltalk and co. don't seem to have any obvious differentiating features to make them the right choice under certain circumstances, or at least not as far as I can tell, and when you add to it the fact that it's harder to find programmers who know it it adds all sorts of other problems for the organization itself.

So if you ever worked on a project where a non-mainstream language (like Smalltalk) was used over a more mainstream one, what was the reason for it?

To clarify: I'd like to focus this on imperative languages, since other paradigms like functional and logic programming language, while not necessarily mainstream, can still be good choices for certain projects for obvious reasons.