Gimp has a notoriously unusable UI. I think that's honestly probably the main reason.
I'm actually more confused by the converse: why do people keep using and recommending Gimp when Krita has existed for decades and is so much easier to use?
It took me a while to switch to Krita, because I used to think it was mainly for painting/illustrating. It took me being unbearably frustrated with Gimp's UI to give Krita a go for basic editing. Never looked back.
For simple image editing with an easy UI, I use Pinta. If I need more advanced features, I need GIMP. I've never really found a use case for Krita personally.
