There's nothing wrong with C++. However, I do my programing in C (without the ++), and would love have something like this available that I could link to my C programs.
GTK+ works fine in its way, but it moves way too fast for my taste. Function x is deprecated, use function y instead. Function y is deprecated, use function z along with function z(1) now. Ok, it's great that you're improving that thing, but not so great for a guy like me who wants to write an application today and use it for the next ten or twenty years without having to re-invent the wheel over and over again.
Since I have no particular desire to learn C++, I now do most of my programming using ncurses to handle the screen, keyboard and (occasionally) mouse. Ncurses is a Text-UI rather than a GUI, but just like the C language itself, it works very well,it hasn't changed in many years, and it suits me fine.
A slow-moving GUI like wxwidgets would be a wonderful thing to add to my toolbox, if it was a C library. *sigh*