Well, maybe I'll get some flack for saying this without providing much evidence. But they aren't a very welcoming group. They have been very insular and tied to RedHat which is driving a lot of the GTK development.
The GTK developers are also incredibly resistant to changes. For example, the code base is still required to follow c89 because some arcane compiler somewhere doesn't support c99 yet and because the greybeards at RedHat thinks it looks nicer with all the variables declared at the top. New developers also struggle with autotools and there has been several calls on the mailing list to replace it with something else (virtually anything) but those suggestions also get shot down because autotools is what the GTK developers know and use.
Just to be clear, I'm not blaming. It is not objectively wrong to be incredibly resistant to change. And GTK is a decent and very polished product. But it is one of the main reasons why GTK doesn't get a lot of new contributors, or resources as you call them.