I think a large part of this comes from the fact that the expressiveness of LLVM’s C++ APIs does not translate well into a “plain old C” style interface. Many of the abstractions and extension points are simply awkward or impractical to expose in C.

On top of that, there is little incentive for contributors to invest in the C API: most LLVM users and developers interact with the C++ API directly, so new features and options tend to be added there first, and often exclusively. As a result, the C API inevitably lags behind and remains a second-class citizen.