I do not think that there a simple answer to that question.
It is as complex as the answer to questions like "Why is A tall, and B is short?" - "Why is A intelligent, and B is not?" - "Why is A obese, but B is slim?" ... and so on.
The answer is always a mixture of: genes, hormones, parental care, environment, peers, education.. and a lot of chance and contingent events.
It is nonsense to be proud of being, say, slim or intelligent or tall ; and being a believer or atheist is nothing to be proud of either. When some atheists say "I do not believe in God or Karma because I 'm so smart!" - it always reminds me of The Donald claiming that he is rich and famous and powerful because he is so incredibly smart and a stable genius.
I'm an atheist myself, but with some other genes, other friends, or some different twists in my life... who knows what could have happened and I would be a monk now.
Our lives are more the result of many little invisible hands working in the basement of our existence, and blind forces pushing us around, than the outcome of our conscious decisions.
And do not forget: Even most of our conscious decisions are the result of the machinery in the basement of our mind...
I'm a serious atheist. Having said that, I was raised in an IrishAmerican Roman Catholic family. I have studied at leas 7 or 8 religions out of curiosity and and positive I made the correct and only intelligent choice there is available. By the way I made that decision at about 11 or 12.
Identity (of any kind) is an abstract construct, just as "self" is. It doesn't "exist" in any material way. Atheists and believers are identical twins. After they die, their bones are indistinguishable. Same for assholes and saints. We all have much more in common than in contrast.
A good message, very well stated. At heart IMO we are all the same thing. We manifest different opinions, attitudes, preferences, etc., but those things are superficial. From a logical perspective mutual respect and love is necessary.
We have to try and understand human egoic tendencies also, and not demonize those who demonize others or claim to be superior. To do that is just more of the same.
Shining the light of awareness is the answer.
I used to be a believer and then many years later I woke up to find that the emperor had no clothes. I was not the average sheep in that I had also studied for the ministry. Some people ask no questions. If you really study the bible you ask questions.
"Why is person A a believer, but person B is an atheist?"
I know this is an expression (which you clarify), but taking it in its literal sense, it suggests that atheists don't believe. This is then like asking why person A is an existentialist, but person B is a nihilist. Possibly because nihilism is literally an untenable position, a practical contradiction. It can only be an intellectual position and not the way anyone can live. '
It's interesting if a 'believer' is a deist, then an atheist lacks belief in a deity. Here the contradiction is on the side of the deist and not the atheist, except I suppose it could be argued that in the atheist's basement a deity hides, mucking about.