Well, unfortunately it IS because the book says so. Different cables and cable assemblies are tested in different ways for different uses. Tray cable is tested for use in cable trays and not for residential wiring.
I know that is not the answer that you want to read but that IS the way it is. Sorry.
I once worked in a facility where welding cable was being used for something to do with a 400 Hertz frequency changer. The welding cable had an insulation rating sufficient for the voltage and the size was sufficient for the current but the inspector said that unless the cable was "listed" for the express purpose that it was being used for it was not acceptable. It cost the company a pretty penny to make the change but that is what they had to do.