why did Enlil create the flood in the Epic of Gilgamesh that was supposed to wipe out humanity?

Answer 1

People were too noisy ; they has also become disrespectful and negligent in their duties/sacrifices......but, ultimately, TOO NOISY

good thing Utanapishtim and his wife were good, diligent people and thereby saved humanity...and all living things...

Answer 2

The part where Enlil decides on destroying mankind is lost. However, there is another similar story about the Gods Enlil and Enki, which might shed some light on the matter.

"The Babylonian Atrahasis Epic (written no later than 1700 BC), gives human overpopulation as the cause for the great flood. After 1200 years of human fertility, the god Enlil felt disturbed in his sleep due to the noise and ruckus caused by the growing population of mankind. He turned for help to the divine assembly who then sent a plague, then a drought, then a famine, and then saline soil, all in an attempt to reduce the numbers of mankind. All these were temporary fixes. 1200 years after each solution, the original problem returned. When the gods decided on a final solution, to send a flood, the god Enki, who had a moral objection to this solution, disclosed the plan to Atrahasis, who then built a survival vessel according to divinely given measurements."