The reasons are quite clear.

Contrary to other bytecode formats, LLVM bitcode is not stable, even across minor releases.

So anyone using it as bytecode format, like Apple, has to keep their own branch, and eventually it becomes too much work.

Microsoft did the same for DirectX DXIL, as did Khronos with the original SPIR definition, thus SPIR-V came to be as replacement, and recently Microsoft also decided to replace DXIL with SPIR-V.