Because in general, when they get the candidates that could fit the position, they get grilled in meaningless letcode interviews, or classical stuff like how many golf balls fit into a plane.
The pool is already small, and gets reduced even further.
And it was only 3-4 years ago (maybe less) that Rust was considered by hiring managers to be in that category, too. Ask me how I know.
I'm going to assume it really means that they can't find people who satisfy some other constraint (location, pay band, "required" degree, experience on some other system or in some industry, etc) and OCaml or whatever.
In any case, LLMs blunt this. Hell, please stop me from opening a tab and starting a new OCaml project right now.