Throughout the galaxy there are hundreds of female Green Lanterns serving the Corps. Among the most famous have been Katma Tui and Arisia - when one thinks of iconic Green Lanterns their names are mentioned. Uncharacteristically Space Sector 2814 has produced four Green Lanterns serving concurrently at the same time - Hal Jordan, Guy Gardner, John Stewart and Kyle Rayner. As far as my understanding the ring or the Guardians will seek out a new corps member when one is needed, but an earth woman is rarely chosen. The source of power for the ring has little to do with the physical makeup of the person wielding it, as it is based on creativity and willpower, therefore a man has no real advantage when it comes to the ability to wield it. On four successive occasions a male has been chosen yet there are many non-males which are chosen across the universe. The reasoning is not the career of the person wielding it either - from earth there is test pilot, Marine, teacher and artist.
I understand that it is a bit of comic based logic thing, the original in the role was a man, and therefore each successive character is easier to cast as male. Also with already three more GLs on earth than usual it would be hard to cast a female here. I also know that there has been Donna Parker, Jennie Lynn Hayden and Anya Savenlovish but these were all for a relatively short time. I am not saying to erase a character or to add more, but the next time the ring is vacant wouldn't an earth woman maybe make a good choice?