Because one of the Rust goals is to improve the security landscape and reduce the use of C and C++.
It is a bit hard to achieve such goal if improving Rust requires to some extent, to keep writing C++.
Naturally some day the compiler might be fully bootstraped, however that implies throwing away the work being done by legions of C++ devs on LLVM (same applies to be front ends being written to plug Rust into GCC).
Indeed, hence why many need to understand that Rust vs C++ isn't a zero sum game, and any improvements to C++'s own security story are more than welcomed.
It is a bit hard to achieve such goal if improving Rust requires to some extent, to keep writing C++.
Naturally some day the compiler might be fully bootstraped, however that implies throwing away the work being done by legions of C++ devs on LLVM (same applies to be front ends being written to plug Rust into GCC).