The actual prelude to Gilgamesh is short. It doesn't go into why Enki was the one to go fight the dragon. It actually doesn't even mention how Enki defeats the dragon. It is just understood from the literary style of the Sumerians that Enki did in fact win.

An is never seen outside of the realm he is ruler of. Ereshkigal is likewise apparently trapped in her own kingdom. Enki is often portrayed as explaining this or that element of magic to Asarluhi. These are little hints from the texts that have been incorporated from outside of the tiny scrap of this myth.

Myths by themselves are generally much more informative if you understand a little bit of the context involved. If you do understand context then a tiny piece of information can be immensely valuable.