I have joined Atheist Nexus just in October and I find, to my utter astonishment, that the atheists are a divided lot, without being in politics. How much united will they remain if you add the lure of power to their minds? Why atheists should want political power? Atheism is a social phenomenon and they have a role cut out for them that is, to rid the world of blind belief, and if possible, give a new social order, post-religion.
This is a big task before us and we should become and remain conscious of this task.
I think the reason has been discussed in various forms in the sense that Christians have a political agenda because they have certain definable opinions - many Christians share the same core ideas.
However, Atheists are only people who not believe in a god. Quite simply, this includes everyone on the planet who does believe in a god - anarchists, buddhists, marxists, non-political peoples, tribes people who have never heard of god, you name it - you get my drift...
Atheism is not a political phenomena; it's simply the lack of belief. It'd be like having a political party for people who don't believe that The Lord of the Rings is real.
However, it should certainly be more possible to have an anti-theist party because an agenda is automatically implied. That said, this would really only work in non-secular countries because many Western countries already have a secure secular foundation (though, to be topical, I wish someone would kick David Cameron up the arse for his comments this week)
All atheists can really do is continue to a) propagate the refusal of faith-based belief b) make sure that religion does not impinge on our social rights and c) hope that enough people eventually catch on.