It has been said that organizing atheists is like herding cats. They are all so independent. There are no doubt many reasons for this.
Many left the church and aren’t too keen on joining another group.
No two atheists are 100% alike.
The phenomenon known as groupthink causes groups to develop or adopt ideological irrational beliefs for the purposes of getting along.
This last one, groupthink, is the culprit in my opinion. Though it may not deter the formation of atheist groups, it does lead to their demise. As the number of independent thinking atheists abandon ship, those that accept the adopted irrational ideologies become more of a majority. Then the group becomes the ideology, and what was an atheist group becomes something else entirely.
This is in process of all humanist groups. Soon, quite soon I fear, they will be ideologues for feminism. It is already a requirement for membership in that group that you support the edicts of feminism.
I remain an atheist. I am independent. I will never join another belief system… including those that are in danger of becoming one. An “army of one” has new meaning for me.