It's quite simple.
There's no empirical evidence for God.
There are other reasons as well. The teachings of many religions are self-contradictory; the Bible, for example, contradicts itself by the second chapter of Genesis.
Religion fails to make a strong case to accept it as true. It has made predictions, which have repeatedly been falsified. Lacking evidence and the ability to make accurate predictions, I see no reason to accept religion's claims to truthfulness.
Atheism, for me, is simply more honest. I'm not making the claim that all people who believe in religion are self-deluded or lying; they may and probably do have good reasons for believing as they do. However, their personal evidence is by nature inaccessible to anyone else. You may well have had a vision of the Father, the Son, and the Holy Ghost, but how can I tell that you have actually had this experience and that you are not a charlatan trying to separate me from my money? There is no way for me to empirically distinguish between your legitimate claim and the claim of a convincing con artist.
Atheism is not a religion; atheism is literally a lack of belief in God. Nothing more, nothing less. There are other philosophies, such as secular humanism, which possess many characteristics of religion, but atheism itself is not a religion.
For me, atheism simply boils down as the position most in accord with the evidence available. Everything else is just window dressing.