Rumor is, the right deal couldn't be reached in Charlotte. So Greensboro was the most effective way to enter the Charlotte market. I wonder if they will look at Greenville-Spartanburg also.
Southwest likes smaller nearby airports that offer cheap landing fees. Passengers certainly don't mind driving 50 miles for a cheap ticket and free baggage check-in.
If Greensboro and Greenville-Spartanburg have empty gates, IM sure they are willing to cut a deal to get Southwest.