Well i think GNOME and even GTK has very few contributors and mainly sponsored by RH and other companies. I suspect they are often getting paid to develop features for subcontracting and other times they want to "modernize" the toolkit and they just write a new API with additional features. We have seen this during DBus introduction, GSettings, Clutter etc... etc... Plus GTK+ is C plus lots of boil plate code to make it "object oriented" which turns to be even a bigger pain when pure GTK+ apps need to update their APIs to catch up with the latest.

But imo this is a general trend on Linux except for the Kernel where Linus and co do a great job to prevent API breakage. The others not so much.