As far as I know, Krita is built on and assumes that the Desktop version of OpenGL is built into QT.
Our Plasma Mobile is using OpenGL ES instead, as requested and recommended by the Plasma Mobile developers.
So you can install pyside2, qt5-base, qt5-declarative, qt5-multimedia and qt5-wayland, replacing the OpenGL ES versions with regular OpenGL versions and have Krita work. This can however have a negative effect on the Plasma Mobile interface in general.
Krita is not built for mobile, so I have no idea why you would want it on your phone.
As mentioned above I was not able to install the recommended packages due to conflicts in the system.
About the Why? question:
I am soon going to travel the world by bicycle, as a pure Linux user I do love the idea to just have my PinephonePro with me as an electronic device. On days where we stay somewhere where is a HDMI screen, I’d like to make basic photo editing (and more). Seems to kind of work with Gimp. I know that this is going to be though with FOSS only, but I want to push the limits hard and motivate everyone to keep on the convergence idea.