It has a large port and excellent air service that efficiently serves the middle of the United States. Those residents are far from the traditional East and West Coast cruise ports. Mexico is conveniently to the south and that's important. Almost all cruise ships in the United States fly foreign flags. Under the terms of the U.S. Passenger Services Act, they cannot sail directly from one U.S. port to another without first visiting a foreign country.
Cruise ships based in Houston or Galveston have an easy run to Mexico.
The fact that they do fly foreign flags also gives cruise ships a prime competitive weapon in their fight to draw customers from land-based resorts. Flying the flag of Panama, Bermuda or Liberia means they are exempt from U.S. wage regulations. Their mostly Third World and Eastern European crews earn lower wages. And while the huge majority of cruise passengers are Americans, and the companies have substantial U.S. headquarters employment, they are domiciled in other countries, which means they pay no U.S. corporate income tax. Then there's the problem of cruise port fatigue.