I think Haskell jobs aren't common because Haskell isn't widely used. Rust is much more widely used but it's also much newer so most uses are from people already in companies starting new projects. Most of those projects are probably still minor and haven't got to the point of needing whole new employees to work just on them.

Where I work we started a fairly big Rust project a couple of years ago and only recently put "Rust" on a job advert.