At that time, and even now. Sumerian is one of the most complex languages on this planet. Only a handful of people are capable of reading it. And even then 99% of what was written was literally receipts, court transcripts, inventories, ect. Very little about their religion and way of life made it into the public realm. Most of it was bought up by private collectors, like the religious fanatics who own hobby lobby.

I never understood that either. "Yeah i dont believe in any of that phony shit, but im gonna spend millions in illegal artifacts anyway." My only guess is they want to buy it up so that others are not tempted to learn about the older more, "pagan" religions.