The neat thing about GAT is that it doesn't really increase the user-facing complexity of the language. Conceptually, users already need to understand generics, and they need to understand associated types. GAT, which allows users to use generics within associated types, doesn't feel like a new feature so much as it just feels like letting two existing features play nicely.
Rust is already a complicated language. Those two features would increase the complexity of the type system quite significantly.
Possibly beyond what is healthy for Rust adoption.