C++ module implementation is a story with a lot of drama, if you ever want to read up on it.
The short summary, though, is that no toolchain yet has a bulletproof implementation, though everybody at least has enough to let people kick the tires a bit.
Because of compiler support
Which suggests (and I don't know this for a fact) that C++ modules are difficult to implement. D's are easy.
I would have made C++ modules be a slightly modified namespace.
C++ module implementation is a story with a lot of drama, if you ever want to read up on it.
The short summary, though, is that no toolchain yet has a bulletproof implementation, though everybody at least has enough to let people kick the tires a bit.