I really don’t know why most atheists I speak with consider atheism a movement.
I wrote about this last year here, and although I meandered around a lot let me try to be more specific. A movement consists of a group of people with a specific goal in mind. They gather together to make their voices heard by politicians and the public at large. They often start out as a grassroots organization and become national or international as they attract more supporters over time.
One example would be the LGBT movement. Ten or twenty years ago, no one paid much attention to them. Now? They’ve organized locally, statewide, and nationally and are a force that politicians across the country, and the general public, listen when they speak. And mostly, they speak with a single voice.
Atheists on the other hand, have very little if any organization of that type. We depend on organizations like AA or FFRF to speak for us. We may even donate a little money to each of these groups. Are we out there marching though, having an “I’m a Happy Atheist” parade? Nope.
If you want atheism to be a “movement”, it starts with us individually, getting together and making our voices heard in our own communities.