What makes Miami surprising is that it’s the type of market the ‘old Southwest’ would never have entered. They already have a huge operation in Fort Lauderdale, and a smaller one at West Palm Beach and while they aren’t exactly the same markets there’s huge overlap. Southwest always favored the less expensive airports, and avoided congested business hubs.
Miami is one of the more expensive airports to operate from in the U.S. This was, if not an explicit policy of pre-merger American Airlines to drive up airport costs, something that their management sat by and was ok with happening – because it chased all the low cost carriers to Fort Lauderdale. That’s changing.