Gimp was designed by open source hackers with no input from professional designers, or seemingly even any interest in their input. It never gathered any kind of momentum as a credible Photoshop replacement for professionals.
And that’s why Blender is getting donations: it’s become a serious part of workflows for all kinds of companies who have a 3D content pipeline. Meanwhile Gimp is still the awkward collection of hackers’ image processing pet projects piled into a clunky GUI that does nothing to enable the things that designers actually use Photoshop for.