From what I understand, GIMP fell behind because it refused corporate donations while Krita accepted them. This lead to GIMP reducing in scope as the 1-3 part-time* developers (at least when I last really looked into it) realised they'd never catch up, leading to people donating less as they weren't satisfied with GIMP's simultaneous underpromising and underdelivering. Meanwhile Krita managed to receive enough money to hire a team of full time developers for several years, leading to better software, to more donations. It's like the poverty trap, but with software.

• Edit: part-time isn't the right word, more like casual