It all traces back to one bad call - the failure of MCAS was deemed to be no worse than "Major" - basically that if the stab trim started doing something unexpected or that the crew didn't understand, they'd simply disable it (which used an existing procedure). "Major" failures don't require redundancy in design. In 20-20 hindsight, that was a fatally bad call, but at the time it must have seemed reasonable. All the problems with the MCAS trace to that - had it been identified as "Hazardous" they never would have implemented it the way they did.