Linux maintainers are unwilling to get rust into the kernel, so some rust folks decided to start writing a new kernel with same ABI. This allows them to make new architectural decisions. An example being their “frame kernel” (something between a monolithic kernel and a microkernel).
If I may say, it’s more legible and the tooling is way better, right off the bat.
In practice, because Rust libraries are always statically-linked, the MPL-2.0 is equivalent to the LGPL in spirit. Meanwhile, because of the static linking restrictions in the LGPL, the LGPL is effectively no different from the GPL. Hence, you’re going to find a lot of open source copyleft projects from the Rust ecosystem preferring either GPL or MPL-2.0, where MPL-2.0 is used in libraries where LGPL would have used previously in C projects. Dynamic linking is essentially going the way of the Dodo.