I believe Atheism is currently popular because the old religions (namely Judaism, Christianity and Islam) all rely on visions of the Universe that have been irretrievably demolished by Science - take for instance the age of the Earth.
Christianity tells us it's 3,500 years old.
Given the clear failure of these systems to survive scientific investigation people do what they naturally do - run the other way as fast as they can.
But they will drift toward the middle again, leaving a few fringe Atheist Extremists wondering what happened to all their friends...
Ultimately "God" or whatever we call it, will remain a part of the cultural picture for the foreseeable future, but it is likely to take a form that is more in line with Science and with experience.
Two systems seem to provide a fairly consistent model already, and their increasing popularity suggests a philosophical shift:
Buddhism and Vedanta (a Hindu sect) where "God" is more of a fundamental aspect of our beings and life is simply a process of evolving to higher and higher levels of awareness.
Atheism, ironically, is a bit too rigid to last as it suffers the same flaw as any Extremist religious viewpoint: it is just too restrictive, too far out there and does very little for people in their everyday lives.
Extremism is a reactionary response, and with the dissolution of whatever we are reacting to, Extremism inevitably withdraws as well.
Put out the flame and the animals return to the forest.