Why Are Modern X11 Tookits Not Written For Xt?

Vinodh D Rajan asks: "Being fed up with the disparate look and feel among the Linux toolkits and incompatibilities between them, I recently started studying the user interfaces available under Linux, something I called The Unified Linux Desktop. For this I started studying how GTK+, QT, FLTK, GNUstep, Motif, Xaw, etc all work. The more I studied them, the more I wondered as to why modern tookits are not written against the Xt standard, since it is the one that has been standardized. Since Motif has been so successful, why not spend effort trying to improve it and provide a common framework for component development, instead writing incompatible toolkits?"

"I am sure that any inadequacies in Xt can be improved. Projects like neXtaw dramatically improves Xaw look and feel. Why not just develop the standards instead of developing newer stuff that is incompatible with standards. Besides the way GTK+'s object system works, seems almost similar to Xt's object system. Apart from cross platform development concerns, what made GTK+ developers write their own subsystem instead of using Xt."