Nginx was written in C. Multithreaded code in a language that doesn't provide any safety rails is hard to get right, and so is async code. They probably figured

Nginx was written in C. Multithreaded code in a language that doesn't provide any safety rails is hard to get right, and so is async code. They probably figured that the complexity of doing both async and multithreading outweighed the benefits that were predicted to be small. Rust's type system checks for and prohibits many kinds of mistakes that are possible in multithreaded code and in async code, so it's much easier to combine them safely.