It’s a result of the Wright & Shelby Amendments that are referenced on the Southwest Airlines website. Basically when the Dallas/Fort Worth Airport was built the city wanted the majority of the traffic out of that airport. An agreement was passed between the city and the airport for traffic to be moved to DFW. Two amendments were passed (due to the fact that Southwest decided to stay at Dallas Love) that limited the destinations out of Dallas Love to the bordering states with the hope that only regional service and airlines would operate out of there, leaving the larger carriers to fly out of DFW.

From what I’ve found this only applies to aircraft >56 seats which means the booming “regional” jet service isn’t affected by this.

I hope this answers your question.