I am guessing, that it is, because imperative processes are very hard to describe properly using type theory. For example: If you have an object, that has some members, that are initially not set, that is a different type than when they are set. So you cannot say that object has exactly one type. After every setting of a member, you have a new type. That's not something that you can statically work with easily. Mutation destroys the type guarantees. The type system can only be on a more limited level, that is rougher and doesn't give as many guarantees, unless you restrict yourself in what you do to your objects. Like for example not mutating them. Or not having nullable members. Things like that.
So I think FP is more suitable for type theory research and pondering, and people who are researching in the matter will not even consider doing that for imperative languages, because they have understood long ago, that these languages lack the guarantees, that you can get with FP, and therefore are not worth spending the time on.
That’s just not true. Rust has a wonderful type system and it can encode mutability, which is a must for anything performance oriented. That is a benefit that FP misses out on. You can still analyze Rust programs with a great deal of assurances. The fact that it’s procedural seems to not matter much at all. An example of a language that allows mutability and has a research-level type system is Flix.
> [...] which is a must for anything performance oriented.
But that's also not really true.
Using functional data structures it is possible to write highly performant software. Check out for example "CppCon 2017: Juan Pedro Bolivar Puente - Postmodern immutable data structures"[1], an impressive talk, in which someone shows their developed editor that can edit huge text files without lag faster than any mainstream editor today. This is enabled by functional data structures. Usage of functional data structures throughout a project also ensures, that parts can be trivially parallelized, which is not true for traditional algorithms, and therefore can easily scale with number of CPUs available. For more on that subject, you might want to watch a few Joe Armstrong talks.
Rust achieves type safety by introducing more language concepts like ownership, borrowing, and lifetimes, to formalize, who has access when and how. It also creates things as immutable by default, unless you make use of `mut`. Unfortunately, Rust's philosophy is shared memory and making shared memory access safe, rather than message passing, which is at odds with FP.
[1]: https://www.youtube.com/watch?v=sPhpelUfu8Q