The airline’s founding was all about its refusal to fly out of DFW airport. Their intra-Texas model depended on short commutes to the airport. The DFW Metroplex has evolved substantially over the past 50 years, and Dallas – Fort Worth it itself a closer airport to much of the population.
And Southwest can’t grow at Love Field anymore, in part because no one can given the reduction in gates there that handed the carrier a near-monopoly. So DFW growth now makes sense, and they’re likely to jump in just as soon as they’re able.