Because C does not support the use of C++ classes. You could inspect each class and map it to a structure, but you would only gain the use of the data members, not the methods. You could write functions to inspect and manipulate the class members directly, presuming you knew the offsets into the class object. This is not a given, but compiler (and compiler option) dependent. You would also have to write functions to replace any operator overloading that was present.
In short, you would have to do almost as much work as if you wrote the GUI yourself. The only difference would be in having source code available to tell you what the original authors were doing with the class. That, of course, also presumes open source.
I'll go so far as to opine that since you don't realize the problems, you will shoot yourself in the foot trying to get around them.
One alternative is to use C++. Another is to use a GUI library with an API that doesn't require C++. To that end, you might want to check if Qt has an interface that supports C. One used to be able to do that with GTK; I don't know if it's still the case.