About 2:10 into an ERCOT video about its history, ERCOT explained that Texas has an independent grid because of its response to the 1935 Federal Power Act. The law gave the federal government authority to regulate power companies that engaged in interstate commerce. So Texas power companies agreed not to sell power outside of Texas, which allowed them the ability to avoid federal regulation.
Texas is connected to both US grids and also Mexico. No one had power to see. Same reasons partially. EPA daily emission restrictions.
The connections were at max capacity.
Classic Texas: