Delays, delays, delays.
Southwest, as a rule, avoids large airports where traffic delays would foul up their nationwide operations. Atlanta, being one of the busiest airports in the nation (or *the* busiest, depending on who you believe), is particularly prone to this, as evidenced by the recent Atlanta snow/ice storm that screwed up operations for airlines across the country.
There has been some noise about Southwest going to Peachtree or DeKalb, but as Flyer732 mentioned, why would they bother with putting their resources towards the ATL market when they'll have to compete with one of the Big Three, *and* another discount carrier? There are plenty of virgin markets getting assaulted by the majors with less competition, and these are the markets that WN is going to put their resources towards developing. It only makes sense that they do this.
Perhaps in 10 years, after WN has maxed-out the medium-tier markets, they'll be ready to start a serious, all-out assault on the fortress hubs. At the rate they're going now, it could take even less time than that. But just look at the long-term strategy: become the strongest you can be serving your bread-and-butter markets, all the while taking traffic away from the majors and weakening them, *then* start cutting into their markets when their weak profit margins can't sustain attempts to run you out of business. Simply brilliant.