In my opinion, the reason atheism has been a minority perspective throughout most of history is because until fairly recently, it was almost impossible for people to ignore the reality of the spiritual world---particularly the existence of demonic entities. If you or members of your family or tribe have a history of interaction with demonic forces, it is difficult to convince yourself that the spiritual world is a fantasy.
The reason demonic apparitions are so much less common today is because of the influence of Christianity and Christian civilization, which encourage orderliness of the mind and intimate that power is obtained through orderliness, structure, and science. As creatures of chaos, demonic entities have difficulty imposing themselves on the minds of people who think like this, even if they have rejected Christ or have no knowledge of Him.
Paganism and animism do not encourage orderliness of mind, so demonic forces have a much more palpable presence in pagan and animistic societies. In such societies, power is believed to be obtained through chaos, as is evident when a shaman tries to control the weather or drive away disease through jibbering and shaking and mixing random ingredients together in a witch’s brew. The shaman is not thinking, "If this then that". If he's thinking at all, it probably resembles something more like, "Jibba wibba glabba-globba. Weeeee! Plip-plip-plip!" A chaotic mind is easy prey for demonic entities.
As people’s faith in the power of science and reason grew, the ability of demonic forces to make their presence known decreased. The voices of demons were silenced, leaving only the voice of God to be heard — but God does not usually speak to people audibly. He speaks through the Bible, and a book on a shelf is easier to ignore than the screams of a demon-possessed man or a demonic apparition hovering over your village. God does not always impose himself. His voice can be easy to ignore, especially if you stop going to church. This has led to the somewhat ironic situation we face today where the widespread adoption of Christian thought patterns has resulted in a considerable number of people who can convince themselves that the spiritual world does not exist at all, an attitude that most of their ancestors would find ridiculous.
It is because of their Christian heritage that atheists are not able to perceive the spiritual world, something for which they should be extremely grateful. Demonic apparitions really mess up your day.
The supreme irony of course is that despite however much the atheists claim to disbelieve in the spiritual world, it still exists and it will inevitably impose itself on them occasionally, for example when they use drugs. In case after case when atheists use recreational substances, they report that they cannot dismiss the otherworldly phenomena they experienced while under the influence as being pure delusion. They cannot help but conclude that they made contact with real, non-human, otherworldly beings. I myself had a drug-using friend who described himself as an atheist, but could also recount many times when he and his family members had been visited by demonic apparitions.