I love the GIMP, use it all the time, find it super-intuitive, and last time I had to use Photoshop I felt like a fresh vim user who couldnt wait to figure out how to get the hell out of there.
I suspect if you surveyed 1000 art professionals, you'd get very few who like or use GIMP. You're going to get a very different result with something like Blender and 3D designers though, which really should make the GIMP devs pause and think what it is that Blender does well that GIMP doesn't.
But I have found Gimp to be wildly inconsistent in the UI. I never remember how to draw a circle, for example, when I just want to add something to a screenshot because you have to use the circle cut tool and then choose a different option which I never remember because it is unintuitive. Yet its easy to have a paintbrush that draws one of those texture patterns that I can't imagine anyone ever using.
As such I then don't use it.
There just isn't a task that Gimp fits.
Most people don’t even think about photoshop UI. Same with Microsoft paint: users can get in and start figuring out what they want to do just by clicking around.
There's hundreds of thousands downloads every month. I'm sure you have better facts at your disposal and will share them easily.