But why

MSVC's module support is ahead of the other implementations IME. Adoption issues have mostly been related to tooling and using modules. Now that CMake[1] has (experimental) support for `import std;` (MSVC and Clang only right now), I think that it is a good time for projects to start investigating modules usage and file issues to compilers for what needs fixing on their side. There's still work to be done on non-compiler, non-build tooling, but I'm working on that too[2].

And I am interesting in modules adoption ecosystem-wide, not just for CMake. I've gone to many build systems trying to get modules support (e.g., xmake[3], tup[4], meson[5], bazel[6]) implemented. There's also the "Are We Modules Yet?"[7] page which aims to track progress.

[1] Full disclosure: I'm a CMake developer and implemented its C++ modules support.
[2] https://cppcon2024.sched.com/event/1gZg8/beyond-compilati...
[3] https://github.com/xmake-io/xmake/issues/386
[4] https://groups.google.com/g/tup-users/c/FhJTA6KAzWU/m/t5P...
[5] https://github.com/mesonbuild/meson/issues/5024#issuecomm...
[6] https://github.com/bazelbuild/bazel/pull/19940#issuecomme...
[7] https://arewemodulesyet.org/