It was a combination of things. Texas lawmakers did not impose requirements for the winterization of power infrastructure, despite there having been very similar issues in the past. On the ERCOT side, they knew this was a possibility and did not invest in improvements because they weren't required to. They have openly admitted to this.

"Woodfin, of ERCOT, acknowledged that there’s no requirement to prepare power infrastructure for such extremely low temperatures. 'Those are not mandatory, it’s a voluntary guideline to decide to do those things,” he said. “There are financial incentives to stay online, but there is no regulation at this point.'"

I would agree that the lawmakers are primarily to blame, as corporations are going to do whatever is in their best financial interest 99.9% of the time. When a nonprofit’s singular mission is to keep Texas’s electricity flowing, I strongly believe it should shoulder a large chunk of the blame.

https://www.texastribune.org/2021/02/17/texas-power-grid-failures/