They were not designed to be launched in below freezing temperature.
Engineers warned the head of NASA that it was too cold. He said that if they didn't launch, they were all fired.
He had a Doctorate in Education. Did not have any background in technical fields.
He also had the unfettered authority to order a launch.
I watched one documentary on that. It wasn’t just the o-rings. Yes the o-rings failed but the hole filled up shortly after it opened. They think it was reopened at altitude by the very high cross winds that further stressed the joints. This is when it burned into the external tank.