Libraries with complex API are usually slow to be accepted by the community, and often libraries that can do less but have a cleaner API are preferred over more sophisticated libraries that have a user-unfriendly API.
The library is accessinble both from plain C and C++ programs. Since the library itself is written in C++ (without use of bloated and bloating libraries like libstdc++ or boost), the C interface is a (very) thin wrapper around the C++ classes. The overhead imposed by the wrappers is negligible.