We strive for simple explanation - as the OP says, the "root cause" - but there can't be a simple explanation for such big phenomena, encompassing hundreds of years and hundreds of millions of people.
I think it's worth noting that there certainly can be simple explanations for big phenomena, even those within human cultures (the concept of currency as a trading medium being a fairly simple idea that has spawned uncountable and unfathomably complex, ever-evolving results). I would agree that the topic here, though, is one that's unlikely to be traced to just one or two such elementary concepts.
As you wrote, even an apparently simple idea like currency is actually incredibly complicated in its historical reality, so complicated that there are thousands of books about it, very active research, and competing school of thoughts that propose completely different and incompatible theories about what are the "causes" and what are the "effects".
So, my opinion is that every simple explanation of any big phenomena regarding human history actually explains very little.
