In my opinion, things such as Qt, WxWidgets, etc., which are implemented in C++ are from the time that C++ was supposed to be the breakthrough language. So these toolkits only targeted C++ in the same way that SWT was mostly thought for Java (although it may have been written in C or C++, I don't know).

This library seems to be implemented in C so that higher level languages can make use of it. C is much simpler to interface than C++. For instance, you mention WxWidgets, but that thing is so god-damn complex that there's still no stable WxPython for pyhton 3..

I really hope that this libui stays as just a simple simple thin wrapper so that we can build complex things no top of it in each language. For instance, python guys may create a declarative framework on top of this while lisp/haskell guys may create a Functional Reactive "thing" on top of it. As long as libui stays simple enough, everyone can extended it in ways that make sense for their own languages..