Texas relies heavily on natural gas power because the price of natural gas went very low in the past decade [1], and Texas has historically favored low average electricity prices over other electrical system attributes. Modern gas generating plants are cheaper to build and cheaper to run than modern coal plants. Coal-generated electricity used to dominate over natural gas in the US and Texas because coal was so much cheaper as a fuel that it was worth building more expensive, maintenance-intensive plants to use it. But that's no longer the case. Every coal plant in the continental US is a legacy plant whose construction started before natural gas prices reached the low levels of recent years.

The same price forces caused Texas to install a lot of wind and (more recently) solar. The federal production tax credit makes wind-generated electricity very cheap for Texans, if somewhat less cheap for all Americans.

[1] https://www.eia.gov/todayinenergy/detail.php?id=44337