Ultimately, the reason secular groups don't spread is actually very simple. They copy AA's surface format (meetings, sharing) without copying the deep structure, which includes the 12th Step. They also falter because they are not really grass-roots organizations where the individual members are responsible for their growth. "Not God: A History of Alcoholics Anonymous" by Ernest Kurtz, which chronicles the evolution of AA for general readership, would be instructive for those interested in this topic.