Part of the reason Krita development has accelerated massively in the last couple of years is that its team decided on a clear mission statement at some point, one very different from GIMP: Krita is a painting app first. The main use cases are painting, illustration and texture-making. That it's also quite useful for cases GIMP traditionally aims at (photo editing, web graphics) are down to toolset overlap between the use cases and a robust, modern tech foundation (colorspace-independent implementation strategies, etc.), but Krita isn't actually trying to compete with GIMP and ventures into territory GIMP doesn't really serve.
Can you compare Krita and MyPaint[1]? (In terms of features, completeness, UI, etc.) From the video, it seems that Krita is a bit more advanced, with image transformations, texture tiling and probably other features. But both programs are digital painting applications, much more than GIMP, which is a generic graphics editor.
Krita and MyPaint share a common interest in implementing things like sophisticated brush engines and color mixing models, and MyPaint is strong there and has been an influence on Krita (perhaps also vice-versa, though I don't follow MyPaint as closely).
One difference might be that Krita existed as a project to write a raster graphics editor for years before deciding to double-down on painting, and did quite a bit of broad foundational work in that space, like aiming to operate colorspace-independently and implementing various general raster editing tools (e.g. a sophisticated layer and adjustment layer system). Krita retains those traits today, which I feel makes it broader in actual scope than MyPaint, which I think directly started out as a personal project to write a stylus-driven painting app and then grew out from there.
Krita bogged down when I tried using a 12,000x12,000 canvas. It also took several minutes just to show the canvas. MyPaint handles frames several times larger with no slowdown on the same hardware. Krita has a better interface overall, but I like to work big.
I think that is only part of the answer. The Krita team is also a lot more compact and has a clearer, more internally defined end goal (it will be interesting to see what happens if its userbase explodes).
Less of the glacial Debian-style politics than have encumbered GIMP never also helped Krita.
Standing on the shoulders of giants certainly makes it easier to see.