The BSA doesn’t allow atheists or agnostics into their club due to the fact we can’t be moral enough for them. The Freedom from Religion Foundation sums it up:
Whatever your views over this change, one thing is clear: Nonreligious boys (and now girls) and their families are not welcome in the Boy Scouts. With 38 percent of Millennials identifying as nonreligious, this attitude is not only ugly and discriminatory, but verges on head-in-the-sand regressive.