Because the vast majority of CSM players wish for a return to a 3.5 system where we have a near infinite amount of options and customisation, both in terms of play style and sheer fluffiness if we wished to depict a specific Legion. As broken as some of the possible combos were, I miss that book so much! I doubt GW will ever return to anything quite like it, which is one of the main reasons why I moved to 30k.