Because the binary interfaces generated by C libraries are simple and therefore universally supported. It's just normal symbols and calling conventions. Conforming to this ABI benefits users of every other language, not just C++.

C++ ABIs are rather complex and much harder to interface with. The existence of concepts like virtual functions and exceptions significantly complicates the implementation of foreign language interfaces.

https://wiki.osdev.org/System_V_ABI https://itanium-cxx-abi.github.io/cxx-abi/abi.html