Because they aren't a community:
[I]t won't be easy for secular Americans to become better organized as a political force, even as they increase in number. The major impediment to that kind of organization is the fact that it is very difficult for secularists to conceive of themselves in tribal terms. Most tribes, whether of nations or ethnicities or sports fandom, can easily demarcate their membership—it's the people who look like us, or talk like us, or dress like us. Tribes organized around religious belief have rituals, sacred texts, and physical spaces that all serve to bind the participants together. Atheism has none of these things—most of the time it's an individual choice, made and kept alone.