I'm guessing that they use LLVM's C++ API, which is not stable between releases. The C++ API changes dramatically from one version to the next, so upgrading LLVM versions typically a big effort for downstream projects. The LLVM C API, however, is meant to be stable. For C++ users, it may make some sense to write a stable C++ API wrapping the LLVM C API.

Disclaimer: I am not affiliated with the LLVM project.