The Internet has a lot to do with it, since it allows information to spread widely and easily, without (much) censorship, and without (much) cost.
Religion thrives on darkness and ignorance.
Before modern times the idea of being an atheist was relatively foreign. I’ve often wondered why then I had a “shower thought”. The increased rate that we consume information has affected the willingness of humans to accept the explanation “because god said so”.
There have been notable atheists in history but few. Traditionally your belief would be the belief system of your father, like he was his father before, and so on. So most atheists we’re atheists not because they turned away from the church so much as because they were never involved in the church. So back then, atheists were much rearer because people just didn’t see it as or know it was an option.
Recently there has been a boom in the “conversion” to atheism. We see almost daily threads and blogs about how to go about leaving religion behind, dealing with the aftermath of parental judgment, how to approach the subject with signification others, etc. The internet provides support, something that before may have been near impossible.
The internet provides a place where it is very easy to find information that allows people to more easily accept that there is no god.
Having a space in which people can engage in critical thinking and the exchange of ideas – on an international level – is necessary to achieve clear thinking. And the Internet provides that space.