It could quite easily be the only declarative language someone was familiar with (or the one most familiar, or the one thought to be most familiar to potential users).

Agree it's not common, don't think I've ever seen it in a non-Hashicorp project, but then declarative languages aren't (relatively!) common.

Alternate tongue-in-cheek answer: if you want a job at Hashicorp? (Or tongue-out-of-cheek: you already work at Hashicorp and are very familiat with it, or looking for a pet project to get very familiar with it.)

Would be interested to hear OP's actual reason though, I'm not criticising the question.