If I had to guess, the actual GCC maintainers[1] had no interest into integrating a very large codebase into GCC which would duplicate a lot of its functionalit

If I had to guess, the actual GCC maintainers[1] had no interest into integrating a very large codebase into GCC which would duplicate a lot of its functionality. LLVM could have been integrated under the GNU/FSF umbrella as a separate project of course. [1] since the egcs debacle was resolved, RMS has had very little control of GCC