Morton-Thiokol and NASA set criteria for safety margins on the Shuttle system. And the Shuttle kept on violating those safety margins again and again. Instead of grounding the fleet or studying the problem further or attempting to ameliorate the problem the safety margins were loosened, deviance was normalized, unsafe operations were normalized. And those who called out the legitimate safety problems with the Shuttle were harassed and derided because their views differed from the orthodoxy (that a Shuttle loss was a 1 in 100,000 probability) and got in the way of their goals (the high flight rate of dozens of launches per year that were necessary for the Shuttle system to meet its promises).

Later analysis of the Shuttle system [2] has indicated several other potential key failure modes that could have caused a loss of vehicle and crew, especially during the early flights. Not just the SRBs and the thermal protection system (the weaknesses which doomed Challenger and Columbia) but other systems such as the APUs, SSMEs, flight software, etc. led to a risk of catastrophic loss as high as 1 in 10 during the first several years of flights, 1 in 20 around the Challenger disaster period, 1 in 50 during the return to flight period after Challenger, and about 1.1-1.3% during the post-Columbia period.

1: https://en.wikibooks.org/wiki/Professionalism/Diane_Vaughan_...

2: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201100...