There are Linux targets which don't have Rust. So the drivers rule is because if we make the mm subsystem depend on Rust then a dusty architecture with no Rust can't run Linux any more. This is tolerable for userspace, realistically your Python document mangling code probably does not in fact need to work properly on a machine nobody has manufactured since the 1980s - but not acceptable to Linus Torvalds.

The expectation is that over the next say, decade, Rust gets more trusted by Linux maintainers and perhaps grows more platform support e.g. via the GNU Compiler Collection, and simultaneously some of the dustier platforms "rust out" of Linux because the few maintainers stop caring about new Linux versions. So one day you can rewrite core subsystems in Rust if that makes sense.