The Sumerian powers rose first, bringing light and life to the cultures of the riverbed valleys. They showed mortals how to put this abundance to good use, which put the sods squarely in the powers' debt. The gods later went on to establish themselves on high. They didn't involve themselves much in the day-to-day lives of their worshippers but were satisfied with ruling and shepherding the people. With the greater power Enlil established as the head of a loose pantheon, the Sumerians were content with their might and let themselves grow lax.
As for the devoted mortals, the city-states of the Sumerians were supreme factions, each venerating the gods with slightly different prayers, icons, sacrifices, and so on. Unfortunately, the splintered worship diffused the abilities of the powers. The gods simply failed to establish any overarching guidelines for their people, and it led to the mighty schism of the pantheon.
Though the Sumerian pantheon is one of the smallest on the planes, it also seems like one of the mightiest. After all, of the six deities in the group, four are greater powers. On the other hand, a few greybeards think the pantheon might soon go the way of all things.
For example, Enki, the greater power of rivers and oceans, recently passed away. His corpse now floats on the Astral, so much spiritual debris. But in truth, it wasn't all that mysterious a death. Enki was a great binder of tanar'ri and was found murdered by two evil Babylonian powers. Nergal (the god of the underworld) lured him to a place on the Lower Planes where the tanar'ri supposedly run rampant. Then Nergal bound Enki, while Anshar (the god of darkness) drove a blade through the river gods' heart. Enki's father, Enlil, has sworn vengeance but hasn't yet found the opportunity.
Most of the Babylonian powers are too concerned with rule and law to kill in cold blood. And where they are orderly, the Sumerians focus on the vagaries of nature and how a body combats that. They're concerned with the establishment of culture, and anything else is just dealt with as it comes up.
The Sumerian Powers