I mean cities in Texas are very spread out. Subways work well in dense areas, and many parts of Texas just aren't that dense. Here's an article about Austin, and it seems people are more interested now than ever. But I want to point to a mention about Dallas:

Basically, the only reason to go underground in the West is when the city can’t get right of way or has to avoid other infrastructure. Dallas’ DART system is the only urban rail system in the Southwest that can claim a subway station. A three and a half mile section of rail runs underneath the North Central Expressway, in order to avoid the right of way conflicts that would come with going through existing neighborhoods. With urban rail projects dating back two decades, Dallas is ahead of the curve in Texas. But Houston’s getting onboard too, according to one of the city’s recent surveys.

We have something similar in my area (also in the west), surface rail is just way cheaper and good enough. Our population is somewhat spread out, so we use streetcars/trams and adjust the lights and crossings accordingly.