why not linux over stock? Ok,i saw the new kde linux mobile,even tho is as usual for only small list of devices,i could see that it has to be installed over cyanogenmode,why u can't install it over stock rom ? we all know that CM actually lose a lot of pieces from the stock  to bring a few impruvements,not to mention that over stock it could benefit of all the drivers,and i have personally a phone that is with a x86_64 cpu and when i looked into some info from the busybox,i sau i686 platform,witch is kind conflictual (i guess),but on this kinda phone,a linux x86_64 should run very well not to mention that would be even better than android itself,that was not build for this kind of cpu,but most of the linux distros today have x86_64 or the amd64 (witch i believe is the same thing),so ? any1 any idea why all of this ?
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.