MSVC does not guarantee a stable standard library ABI, so those that need to target it are out of luck. In linux land, the switch from libstdc++5 to 6 still haunt the memories of many, although it was quite a long time ago (gcc 3.4) and the ABI has been stable since.

In practice the reason that many projects provide a C ABI is that they want to interoperate with other languages and the C ABI, being extremely minimalist, is the least common denominator.