This would be a personal opinion so take it for what it's worth. I doubt that on a relative basis we have as many people left today who are as convinced in the concept of heaven as compared to a thousand years ago. Just think of all the instances where you would give someone a choice to denounce their religion or get burnt alive, and people chose the latter. Comparing that on a relative basis, I think fewer of those choices would be made today.

As to why, a thousand years ago you didn't really have a strong counterthesis to the Intelligent Design thesis. As the Evolutionary principles became better understood and more accepted, religions have had to shift their position a bit (initially the Earth was the center of the universe, etc).

Long story short, even if people do believe in heaven, they wouldn't mind having a small hedge, just in case.