I thought that, too. At least for the most part. Pittsburgh, for example, has a declining Catholic population. A lot of those cities are, as you say, “old standbys” of Catholic American past.
Nevertheless, San Antonio has always struck me as a genuinely, passionately Catholic city. And I was really surprised to see Denver on the list, as it is really not a major Catholic population center. Denver’s place on that list is probably due almost entirely to the incredibly vibrant new evangelization emphasis going on here. My wife and I are considering a move to Texas, but I don’t think it’s possible to find a diocese as incredible, orthodox, and vibrant as Denver. Denver is actually creating a new Catholic school next year. Our former Archbishop Chaput is, on the other hand, dealing with massive school closures in Philadelphia. That’s a pretty stark difference.
I’ve lived in St. Louis, and loved the fact that they have Mass at the airport. The city truly is a Catholic gem, and continues to be that way. The Catholic school system is also very strong.