Sincerity of course. There's so much work to do on GIMP, especially with GEGL integration and non-destructive editing, and too few hands to work on it. It would actually deserve 5-10 full-time developers given the project size, but 1 for the moment is good enough.

If you ever tried slipping a few line of code for a side-project or a free/open source one, you know how hard it is to focus when you have such limited time and energy.