Likely because people in the Ruby community tend to favor solutions that allow them to continue writing Ruby. In the context of the OP, Rust is being used only to augment Ruby, to complement it, not to outright replace it. Adopting Swift or Crystal would involve rewriting much more code from Ruby into the new language.
Look at it this way: for these programmers, the choice isn't between Rust and Crystal, it's between Rust and C. And for people who've never written systems code before, Rust is enormously friendlier.
> Swift as well as Crystal can be used in here too
Can you elaborate? I don't know of any way to compile Swift or Crystal code into a library that exposes a C-compatible ABI, and I can find no documentation as such.
Look at it this way: for these programmers, the choice isn't between Rust and Crystal, it's between Rust and C. And for people who've never written systems code before, Rust is enormously friendlier.