Not sure, but, I would bet it has something to do with the body mounts and attachments to the body (fuel filler hose, steering, etc). I have seen X's with the 3+3 lift, so it is possible to do, just probably not bolt on. They advertise their stuff as bolt on, so if any additional mod's are required, they will say it 'wont work'....
BTW, I had that kit on my kick for years and LOVED it!!!!
Zig