Earlier this year journalist Andrew Zak Williams interviewed about 30 public figures who believed in God. Their explanations for their belief were published in a New Statesman article. Now Williams has followed up his earlier piece with another NS article, this time presenting the reasons given by prominent atheists for why they don't believe in God.
As for me, my answer has been covered by many of the NS atheist respondents, with greater eloquence. But for what it's worth, here's why I am an atheist:
Lack of evidence for gods and the supernatural, the obvious human-centric (usually male) artificiality of religion, the correlation between levels of poverty, education and security and levels of religiosity that suggests a mundane - not divine - explanation for religion, the evidently indifferent, amoral universe of randomness, accident and chance, and a personal preference for reason over faith.