Rust promises to never break or remove anything from the standard library, and Rust also wants to be a language usable for the next 40 years. This means that it has to be careful not to add stuff that's going to become outdated and deprecated cruft that they'll have to maintain forever.
Rust has already made a mistake of including a channel implementation in stdlib, which ended up being inferior to crossbeam-channel. Now, as you can even see from TFA, there's a dilemma whether to use the slower, less flexible one from stdlib, or a crate. If there's only a crate, there's no dilemma :)
Rust promises to never break or remove anything from the standard library, and Rust also wants to be a language usable for the next 40 years. This means that it has to be careful not to add stuff that's going to become outdated and deprecated cruft that they'll have to maintain forever.
Rust has already made a mistake of including a channel implementation in stdlib, which ended up being inferior to crossbeam-channel. Now, as you can even see from TFA, there's a dilemma whether to use the slower, less flexible one from stdlib, or a crate. If there's only a crate, there's no dilemma :)