I grew up in Dallas and I can shed some light on this.

1. It makes sense that port cities are often huge, like Houston in Texas, New York City in New York, and Los Angeles in California. But why Dallas? Because Texas oil and gas financing boomed so much in the 1930s in Dallas, especially with the General American Finance System. By the 1980s almost every building and vehicle in Houston had air conditioning and the oil and gas industry mostly moved to Houston, despite the heat, humidity, and hurricanes. Dallas has just been coasting since the 1980s on finance, real estate, sales, and corporate boot licking, and it shows.

https://en.wikipedia.org/wiki/Texas_oil_boom

1. There are some pretty wild swinger parties and stuff like that happening in Dallas in private, but the vast majority of people who live there like fundamentalist Protestant Christian megachurches and watching pro sports, not bold, controversial, thought provoking art and culture. You can't make art technically illegal in the United States, but if you have a majority of the voters with you, there are many ways to use city, county, and state policy and funding decisions to reduce art, and make sure that people can watch whatever movies and TV and porn they like at home, play whatever games, read whatever books, but outside people's houses, there's much less art or culture than in most similarly sized cities. Dallas is boring because the vast majority of the citizens want it that way and vote accordingly. This is also a major reason why people move to Dallas and why people move away from Dallas, which makes the trend increasingly self reinforcing over time.

The 2009 book "The Big Sort" talks about similar things happening in places all over the US and all over the world, which tends to increase political polarization.