At least for me, Rust gives me the parts I like about FP (sum types, generics, type inference, limited mutability, first class functions) without the annoying parts (bad tooling, poor documentation, odd syntax, laziness, mediocre ecosystem, very circuitous routes to mutability and IO).
Basically I suspect the majority of FP users liked the type system and general guarantees of FP but didn’t care about the ideological parts like purity or laziness so when a language popped up that gave you those guarantees but also had better UX and could bind easily to C or C++, they hopped on.
Also it’s much easier to sell Rust to management as C++ but with less bugs. Who doesn’t want less bugs?
All of that describes me. I'd add that I really like rust syntax. Ok, maybe not the syntax itself, but the semantics of the decorations. It trades a little up front pain, in the form of "cmon compiler, why do I need to tell you about this?!", to save a lot of pain later when revisiting mostly forgotten code, "oh i see, the return pointer is tied to the lifetime of argument 2, not the others" (as opposed to "WTF are the invariants here again? time to go hunting").
Basically I suspect the majority of FP users liked the type system and general guarantees of FP but didn’t care about the ideological parts like purity or laziness so when a language popped up that gave you those guarantees but also had better UX and could bind easily to C or C++, they hopped on.
Also it’s much easier to sell Rust to management as C++ but with less bugs. Who doesn’t want less bugs?