East-Europe countries are very atheistic because of communist indoctrination into atheism since the youngest age. Similarly, very religious countries are because people are indoctrinated into being Christian, Muslim or Hindou.
Parents, public figures, and countries tell you to be of X or Y religion, or to have none, 95% of the world chose its beliefs in that way. If you're born in Czech Republic, chances are you'll become atheist, if you are born in Saudi Arabia, probably Muslim, if you're born in Spain, Christian.
It's all about being indoctrinated into believing something is. Almost no one look at other religions, faiths and beliefs (or disbeliefs) in an enough open way so that you can make your own choice out of long and rational thinking.
Actual projections say:
The number of Muslims will nearly equal the number of Christians around the world.
Atheists, agnostics and other people who do not affiliate with any religion – though increasing in countries such as the United States and France – will make up a declining share of the world’s total population.
The global Buddhist population will be about the same size it was in 2010, while the Hindu and Jewish populations will be larger than they are today.
In Europe, Muslims will make up 10% of the overall population.
India will retain a Hindu majority but also will have the largest Muslim population of any country in the world, surpassing Indonesia.
In the United States, Christians will decline from more than three-quarters of the population in 2010 to two-thirds in 2050, and Judaism will no longer be the largest non-Christian religion. Muslims will be more numerous in the U.S. than people who identify as Jewish on the basis of religion.
Four out of every 10 Christians in the world will live in sub-Saharan Africa.
However, I think they just use predictions using reproduction and average conversion rates, not really really taking into account the possibility of conversion-rate changes with time, specially from religions to non-religious categories, thus Atheism could still increase a lot in the next years...http://www.pewforum.org/2015/04/02/religious-projections-2010-2050/