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...
I'm not sure how anyone could say that with a straight face.
Serious events happen with commercial aircraft at a rate of about 1 in 1M. Figure that a rocket is inherently 100X more dangerous. And that the lack of repetitive experience with rockets bumps the risk another 1:100. Meaning you build a lot of aircraft and fly them all the time you get good at knowing where the design holes are. Also consider that each serious failure when mitigated improves the odds of the rest of the fleet. You don't get that when you're only flying a dozen flights a year.
Est risk: 1X10-6 X 100 X 100 gives 1% chance of loss.
Notable the Apollo program flew 15 flights 3 had serious failures[1][2] and 1 near loss event (Apollo 13). For consideration the Saturn V was a well tested conservative design which the Shuttle was not.
[1] Apollo 6 and 13 had early engine shut downs due to failures of the S-II engines.
[2] Apollo 12 lightning strike took out the command module computer.
There was also a bit of self-delusion going on. The Shuttle system was, by design, the backbone of spaceflight (manned and unmanned) in the US at the time. On the one hand you could believe that the Shuttle system was a modern miracle, fully capable of achieving (or nearly so) its design promises of cheap and ubiquitous spaceflight, ushering in a new space age, including the launch and assembly of a next generation space station in the near future, and possibly including the realization of manned missions to Mars within the next decade or two. The competing view, that the Shuttle was a risky launch system that could never achieve its design promises even within an order of magnitude, was a vastly depressing (though in retrospect realistic) one. Holding that view meant that we would have to go back to the drawing board and spend maybe another decade building a new launch system that would reset us back to the way things were in the 1960s, and then we'd have to slowly crawl our way toward incremental progress. That was a very difficult truth to accept, ironically more difficult the more you were invested in space exploration.
Sometimes reality is a tough pill to swallow.
There was some coverage of the relatively high risk values in the popular science press well before Challenger happened.
