One reason is that Google and other members that agree with their position, want to be able to export macros from modules, instead of using the multiple file approach, meaning #include for macros and modules for everything else.

A simple solution is already possible with Microsoft's implementation, but they don't like it, it seems.

    #ifndef LEGACY_HEADER
    #define LEGACY_HEADER

    #include "legacy_header_macros.h"

    import new_header_module;

    #endif

Right now there isn't much public information how they will sort this out.