I'm assuming that there never was a global flood, since it would be impossible given the earth's water content and distribution. So why make up such a story or include it in canon? I could only come up with one answer, and found it a compelling one. What would we expect to happen following such a discovery in the ancient world?
I would say that at the time the flood account was authored, there was not a concept of a global world. I would imagine the world at that time was a much smaller place.
As to why the author would choose a flood scenario, I would suggest that flooding would be a large scale disaster that the author and the author's community would be familiar with. If the author wanted to imagine a world-ending event initiated by the gods, poetic license would permit the author to simply scale up such a familiar event.
If the author lived in an active volcanic region, or one with seasonal hurricanes or frequent earthquakes perhaps one of those would be used by the author as the mechanism for world destruction by the gods.