why not linux over stock?
It is probably possible to get it working on other ROMs, however manufacturers like to add their own spin on the usual AOSP, and some of these changes can be really low-level, including changes to the kernel which might make it incompatible with what is essentially a new OS userspace being installed (see for reference: Ubuntu Touch)

As for why Cyanogenmod, it is the most universal ROM distribution that supports this kind of development and actually sees widespread use. Unlike stock ROMs, Cyanogenmod ports to other devices do not see drastic changes to the architecture that would change compatibility compared to other Cyanogenmod releases (hardware limitations excepted). Therefore it makes sense to target this Android distribution, especially if Cyanogenmod is supporting low level features that KDE Mobile may be using, but the various iterations of AOSP may or may not allow.