Why wouldn't you? The size of the community is definitely a significant advantage of Rust of OCaml, but if you want to use either then you need to be comfortable with reasonably niche things. Other than that, what is there? I don't believe multicore is relevant for the vast majority of projects (for instance those where JavaScript or Python could be plausibly used).
On the flip side, OCaml has a GC and Rust does not. For any other language and most usecases, that would be a dealbreaker. It's very cool how Rust has made it less so, but I'm not convinced it is magic enough to make it insignificant.