Summary

Because Open-SSL is a C library that is hard to build especially on Windows

Detailed Explanation

Building Rustpython with SSL enabled is a pain at least on windows which requires you to manually set up stuff beforehand, VCPKG is apparently the easiest way to get Open-SSL. Rustls, on the other hand, is written entirely in native Rust as far as I know and will compile just fine without you having to do anything. It's also statically linked and apparently might be faster than Open-SSL anyway.

Drawbacks, Rationale, and Alternatives

A possible alternative is the native_tls crate, which abstracts over platform-specific TLS implementations. I believe it works automatically on Windows, but falls back to Open-SSL on some other platforms.

Unresolved Questions