The short answer is, because all the facts and evidence demand that conclusion. We can break down the argument into several parts:
1. The theory of evolution disproves all super-natural theories of creation.
2. The is absolutely no evidence for any of the world's religions.
Here are the major influences on my thinking:
1. Stephen J. Gould. His wonderful popular books about paleontology, evolutionary theory and the history of science showed me how revolutionary evolutionary theory is. They also showed me how beautiful and powerful science is.
2. Sam Harris, and especially his book, The End of Faith. This book exposes the absolute lack of evidence upon which all religions are based, and confronts the real danger that religious ideas pose to the world.
3. The scientific journals, Nature and Science. These are the most exciting, the most informative, and the most important journals in the world today.
Once I saw the true beauty and power of scientific thought, it became impossible to believe the drivel that we know as religion and most (not all) forms of spirituality. It became clear that religion is a crude form of political manipulation, with fear of eternal damnation as its only real weapon. There is no evidence whatsoever for such fear. To give up religion is to give up the fear of death.
Seen in this light, the new atheists could be said to be stating the obvious. However, there is a lot more that can be said on the topic of replacing religion with science. I recommend the following YouTube videos as a starting point.
Richard Dawkins: Militant Atheism
Sam Harris vs an evangelical (The full debate is here.)
Sam Harris: Death and the present moment
Sam Harris: Science can answer moral questions
Harris and Dawkins are both respected scientists. I recommend all of Dawkins's books.