That was a political decision. Having your own grid means that you also have your own power frequency regulation, and importing power from a different network is not as simple as connecting wires - you need to connect them using DC instead of AC. These transfer points (interchanges) are sized just for temporary power import or export, you cannot use them to power the whole Texas grid. You can see more details about the interconnects here: https://www.eia.gov/realtime_grid/#/status