Untested technology at the time.....
Remember folks, without the aid of whiz-computers, a destrutive type of testing was done to hardware at the time, all all up approach to flight testing was novel that the time and untested, so let's look at the big picture and count our blesssings additional goofs did'nt happen with the STS, just minor glitches that culminated with the loss of 2 orbiters. Besides, what's the odds of losing a shuttle? 1 in 100 launches?