How so? The likely reason is that a C lib is usable from many other environments. Look at ncurses for example. Heavily used from many languages, and written in C.
Another reason is that the underlying tools used by this library are likely in C as well, so it's probably the easiest language to use for the task.
C is easy to package and distribute. I once built a Python + GTK application for a school project. On Ubuntu that was no problem at all, but on Windows it was a complete mess because I had to bundle Python + GTK for Windows + all their dependencies. I think the result was around 200 MB of dependencies and ~100 kB of actual Python code. C + libui would have been much better in this regard.
Another reason is that the underlying tools used by this library are likely in C as well, so it's probably the easiest language to use for the task.