The Texas Interconnection is tied to the Eastern Interconnection with two DC ties, and has a DC tie and a VFT to non-NERC (North American Electric Reliability Corporation) systems in Mexico. There is one AC tie switch in Dayton, Texas that has been used only once in its history (after Hurricane Ike). On October 13, 2009, the Tres Amigas SuperStation was announced to connect the Eastern, Western and Texas Interconnections via eight 5 GW superconductor links.
It’s a matter of capacity. ERCOT only has 1220 MW of interconnection capacity across 4 asynchronous links [1]. Compare this to e.g. California ISO which seems to have about 10,000-15,000 MW of synchronous interconnection capacity [2]. Neither is adequate to fully support historic peak demand in the system [3-4], but California has much more flexibility in emergencies.
https://wikipedia.org/wiki/Texas_Interconnection#Ties