The reason is that their culture forbade them to ignore God.
Please do not overestimate the role of high intelligence. It is a tool, and it may be better that it be less than that it be put to evil uses.
I don’t think it’s overstating it to recognise that in the modern world, it’s a very important factor. When it’s lacking, the only thing which comes close to making up for that lack, even in a bad way, is superior brute force.