Because developers are lazy. Including myself

I did some apps using Gtk+ some years ago and nothing wrong with this toolkit.

But now I prefer Qt(Quick) it has more potential even for commercial apps etc...

And even if we prefer opensource we have to realise some apps will never be open and commercial devs prefer Qt over Gtk+ mostly because Qt is more "complete framework". That was probadly one reason Nokia dished Gtk/manyclibs in favour of Qt...

Just take a look at N900 and Maemo5 its WAY easier write apps using Qt(Mobility) instead of talk to lowlevel libs in C on Maemo5.

And yes I know there are SEVERAL bindings for Gtk+/Glib but in a way this gives more headache cause then those bindings has to be "extra" loaded in memory. Atleast in case of C -> C++ bindings its a drawback. In case of Python I guess its the same(drawback)...