It isn't a question of backwards compatibility, it is just way easier to iterate / evolve in a crate than it is in stdlib.

For example, Tokio has the concept of "unstable" features, which have stronger stability guarantees than Rust nightly. First, you can use them with the stable rust compiler, second we guarantee that unstable features will not break across patch releases. This may seem small, but it lets us experiment with new functionality and get real world usage. Many of our users cannot use Rust nightly but can use Tokio unstable features.