because GIMP UI is terrible and the GIMP team has hard time questioning their UI choices. It was the same thing for blender for years, artists were telling the team that toolbars on the bottom of a window was a stupid idea, that the 3D view needed a visual nav gizmo and so on and so forth only to be rebuked by "gatekeepers" opposed to any sort of change. Yet Blender really took off with 2.8, only when it did its visual overall...

If you're going to write a software for artists, design the UI for artists, not developers... "just learn the UI" doesn't cut it. The problem isn't resource oriented, it's "we don't want to change anything because we think there is nothing wrong with the UI".

While every software is different, there are rules to UX/UI and GIMP is breaking too many of them. Also the bar to contribute is C and GObject knowledge which is quite high. Blender UI can be modified with Python so it's much easier for a designer to make prototypes.