Because Southwest's business model, especially back then, is short haul flights between cities that are just far enough from each other to make driving inconvenient. Guess what else is a good and way more convenient transportation method for that?

Because they had a nice revenue stream running short-hop flights between those cities and didn't want high-speed rail cutting into it. Routes under 300 miles or so are the sweet spot for HSR where you can actually save time going from downtown to downtown instead of dealing with the airport rigamarole.