The biggest reason is that it’s much harder to write prototype code to test out an idea to see if it’s feasible and feels/looks good enough. I don’t want to be forced to fully plan out my code and deal with borrowing issues before I even have an idea of if this is a good path or not.
There are options for this with Rust. If you wanted to use pure Rust you could always use unsafe to do prototyping and then come back and refactor if you like it. Alternatively you could write bindings for C/C++ and do prototyping that way.
Though, I will say that this process gets easier as you gain more experience with Rust memory management.