Good question. I guess we all carry baggage of one sort or other, and letting go of such things can be difficult. Many of the atheists here come from theist backgrounds, and so old habits die hard. Personally, I don't come from any form of theism, so for me it's natural to be a bright, as I know no other way. As the last of 8 children, even Santa was a joke to me from as early as I can remember. Even during my years as a Marxist/Leninist in the Communist Party of Canada, my actions were more from revenge for wrong done to me by the state, than from the false belief that we could educate the masses and create a worker's state.
I guess I'm not one who can really answer your question very well. Perhaps our former theist comrades can better than I.
Well, this is the thing. I do notice quite a few atheist that are not brights. It's just that I think it is kind of weird when atheist end up going a long with some mystical power, or some far out conspiracy.
Many atheists join groups thinking that they will be among people of like minds. From my experiences, this is just not the case. It isn't even true that all atheists are against a Theocracy. This was evident when many atheists voted for a third party candidate when they knew that they were taking a vote away from the only candidate that would protect the Constitution by appointing a less conservative minded Supreme Court Justice. It appears that many try to define us as a group by the one thing that we don't believe. This is ludicrous. Many don't believe that immunizations cause Autism. That doesn't mean that they have anything else in common. I, also, think that third party political movements intentionally promote such an idea to draw in non believers. There are just too many different types of biases that afflict the American public.
Brights are a subset of atheists. By their own definition, they seem to be pure. That is, they not only not believe in god(s), but they don't believe in anything that can't be explained in natural terms. Unfortunately, their definition is misleading to the less informed, "naturalistic worldwide view" and they have chosen a name that has more than one meaning, and many, because of pettiness, gravitates towards the one definition that doesn't really apply. I am not an official member of the Brights, but on several occasions, I have found myself defending their name to other atheists. As it turns out, I'm much more likely to define myself as a Bright than any other form of atheism. I am researching the group to make this determination.