This was a failure of ERCOT management and a failure of public policy that put electrical reliability at the risk of unreliable primitive energy (i.e., wind and solar).
The reserve margins were adequate, but they were a mirage. In 2016, ERCOT claimed a reserve margin on nearly 20% would be in effect in 2019. They got to this number by using average seasonal output of primitive renewable energy. But, the wind doesn't always blow and the sun doesn't always shine. In fact, the 2019 reserve margin was less than 9% and at times less than 2%.
Texas nearly had a blackout in 2019, so the problem was known. For a good write up on that near disaster, read ERCOT’s Phantom Reserve Margins Spell Trouble for Summertime by Jason Isaac of the Texas Public Policy Institute.
Isaac also has the best write-up of the recent black out, which is The Federalist: Texas’s Blackouts Are The Result Of Unreliable ‘Green’ Energy.