One of those reasons is that now people know things they didn't know back then. (Like age of the Earth, dinosaurs, evolution, etc.) In the past people could believe religious explanations because they honestly seemed to them as the best explanations. Now you have to actively suppress education, or support some kind of doublethink, or invent a new interpretation of the old writings -- these are all modern elements that the honest ancient faiths didn't have.
The ancient religious people believed that evidence would support their faith. This is why in the past many religious people were also great scientists: they believed they were studying God's work, thus contributing to understanding of God. The modern religious people know that evidence is the enemy of their faith. On some level they are aware that their religion is a "noble lie". Sufficiently intelligent people realize that noble lie is still a lie.
Beware triumphalism. The gods not being real has never stopped them for long before. Or rather whenever one religious sensibility becomes untenable another springs up. Its the way we work. One particular mythology losing its grip on people's worldviews won't stop people from having religious experiences and building cultures and communities and mythologies around them.
An understanding of reductionism isn't a stop sign to this either. To use an example I have spoken about before, one of my good friends being an atheist materialist reductionist doesn'...
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.