It seems to me that while Rust has a stdlib, there is also a de facto set of "standard" libraries that the ecosystem consistently turns to. serde and regex are the two that stand out in my mind. I'm not sure that incorporating them into the standard library would bring anything to those crates now, while only bringing additional constraints.

That said, some projects that had a similar status, like once_cell, are being incorporated into the Rust standard library, while others, like regex are now owned by the Rust team, but still managed as a separate crate. I'm not really sure why they have taken different approaches between the two.