From what I understand and know from reading, it is really hard to sell krita on ios or apple store. Apple doesn’t like Free software programs so the license is incompatible with their closed garden goals.
Even if it was it would be hard to port it since apple behaves like it is out of this galaxy and unique, it doesn’t follow open standard like OpenGL and tries to implement its own system (metal), it is my way or highway, something which small projects often made by volunteers can’t do unlike the big boys and corporate.
It would be nice to have Krita on ipad too since we have it on beta for android, but apple customers need to tell apple to take it easy with Free and Open source people and make their life easier. Similar problem is for Krita on macOS. The developers try really hard to make it work and try to provide good experience despite apples shenanigans.
P.S. sorry for ranty answer, but its just that apple tries hard to irritate people who are not after profit.