As you may have heard, Texas is big. But much of the population is concentrated Dalas-Fort Worth, Houston, San Antonio and Austin. Although those cities form a nice triangle and are fairly close together compared to the rest of Texas, there are still significant distance between them: Dallas to Houston, for example, is 420km. Austin to Dallas is 300km.
Still, that is a high density of racing. France has a population of 65 million people and no major race in 2013. Metro Sao Paulo has a population of 20 million, and will have 2. However, Indianapolis only has 1.7 million people and yet will have 2 races. Greater Detroit will also have 2 races despite their economic depression.
By the way, the Austin F1 race is getting a 25 million dollar state subsidy, per year, for the next 10. That is about $250 per ticket!
Formula 1 season (Wikipedia).