Here's a study of the top 100 HSR city-pairs by an independent researchers.http://www.america2050.org/pdf/Where-HSR-Works-Best.pdf
From that link,
"Although one Texas city pair made it into the top ten in the index (Dallas-Houston), the other major connec- tions in the Texas Triangle are further down on the list (Austin-Dallas: 45th; Austin-Houston: 54th; Houston-San Antonio: 56th: Dallas-San Antonio: 70th). These corridors tended to be ranked lower than the city pairs in California (six California city pairs were ranked in the top 25) and the Midwest (with city pairs including Chicago, Detroit, Columbus, Cleveland, and Pittsburgh), which all appeared multiple times in the top 50 pairs. Although these Texas corridors scored well in overall population, length of cor- ridor, and economic activity, the lack of (or limited) existing local and regional transit systems in these cities reduced their overall rankings. City pairs with at least one city with local transit and commuter rail systems tended to populate the top 100 city pairs. Corridors which included two such cities including New York, Washington, Philadelphia, Los Angeles, and San Francisco all can be found in the top 10."
So, the real reason why Texas Central is wanting to build and operate the HSR line between Dallas and Houston and not to Austin and San Antonio is on real data based upon research. All the demographics favor it, based upon the 4 main criteria; population, distances, economics, and transit availability.
And let's face facts, Austin and San Antonio don't have great transit systems as compared to Dallas and Houston.