Because the linker provided by the system itself is the one that's guaranteed to be the most broadly compatible. For a toolchain to begin distributing an alter

Because the linker provided by the system itself is the one that's guaranteed to be the most broadly compatible. For a toolchain to begin distributing an alternative linker is a lot of work on every supported platform (future versions of Rust may sidestep this issue by making LLD the default on certain platforms, which they get "for free" by dint of shipping alongside LLVM). But also, because there are about ten people on planet Earth qualified to write a production-grade linker, which is why LLD still isn't broadly recommended yet.