Rust’s stdlib is intentionally minuscule so that breaking changes can be made to important crates. Once a crate becomes part of Rust, no breaking change can ever be made to it, which is a pretty high risk situation (if chrono had been added, jiff would not be able to). Better would be for Rust to simply fund development of critical crates and let the marketplace work out which crates those are.

I think that Rust's tiny stdlib is one of the most glaring flaws of the language. I don't think that there's actually any significant upside, because people can always just use a crate if they want. On the other hand, there's significant downside because not everyone has the ability to install crates. The stdlib is the one thing that is guaranteed to be present, and it should have the most commonly used features developers will need.