It is about integration, serviceability, consistency and driver quality which is where they excel, with full control and in house production and lineup that doesn't change every few years. And in Northwards case of course also about consistent results from room to room, which is undoubtedly easier when using the same speakers everytime. The ATC lineup can be ordered specifically for soffit mounting with external amp, straight edges, and their LF response is made with soffit mounting in mind, whichis what you want when building this kind of system. That is also why a freestanding pair often sounds thin and lacking LF, the install version doesn't have any EQ settings on the amp, whereas the freestanding/integrated amp have a +6db gain pot for the LF. They are designed with the LF gain from soffit mounting in mind, and behaviour is predictable and consistent. All of these "SOTA" speakers are not made for this kind of system, they are made to intereact as little as possible with the common listening room. They can not be integrated in this manner, as that is not what they where made for. It doesn't matter if it's a better speaker on a Klippel, because in this scenario they are not "better", or even usable.
There are acoustic problems completely eliminated with a good soffit mount system, and then the advantages of Kii or C8 dissappear. Few speakers are actually even good candidates. There are not a lot of choices out there - if you have heard a pair of the big Genelecs made for Soffit mounting, I think you will know why they are generally not used anymore. They are crude and unpleasant to the ear. The only other real alternative is Quested, as PMC has a weird voicing with excessive HF. Quested is a much smaller and more niche company, using off the shelve Volt drivers, and LAB amps.