LLVM is a much more mature and modern compiler by basically every measure, and importantly, a native cross compiler unlike GCC.

One copy of LLVM can build for many architectures vs needing one GCC toolchain for every architecture you wish to target.

Also modern languages like rust require LLVM so we need it anyway, and the kernel has rust now, so why not make the most mature compiler stack the global default? Using two different compilers for the kernel would be begging for problems.

I am most optimistic for the Fil-C fork of LLVM being a default some day where we get compile time memory safety too.

We do maintain gcc in tree for legacy languages like Fortran though.