Why to switch to other toolkits?!
I not understand what don't good in GTK+.

And also - why to keep GTK+ 2?
GTK+ 3 avilable in all distro, also in older distro,
it also work in Windows and also in Mac. this
because GTK+ 3 released before almost three years.

Because many distros are more than 3 years old and still supported.  Geany has users (and developers) in commercial and academic environments which will often be constrained to older (but still supported) versions of Red Hat, Scientific Linux, Suse etc.

Its fine for hobbyists to want to play with the latest, we only have one PC to change over, but institutions often have hundreds or thousands and have all sorts of specific software to ensure still works when they upgrade.