The original Boeing 737 entered airline service in February 1968 with Lufthansa. In 1968 they didn't do digital processing and Kalman filtering of flight instrument data. In other words the MCAS system is based on recent tech, the MAX is to some extent a hybrid of old and new, and thus the type certificate which was based on validating an entirely mechanical design should not have been extended without a very painstaking review.
After the pitch-down incident of QF72, the fault was pinned on bad processing of AoA data.
The final report dwells in detail on the *novel* difficulties posed by the validation of digital systems and the problems posed by complex systems. Incorporating a new system for attitude correction (MCAS) and digital processing in the 737 raised both of these issues, and allowed new problems to creep unnoticed into an old design.
Let me try and be clearer - a pilot knows that if your airspeed falls too low, the plane stalls. An engineer knows that if you add a computer to an existing design of a mechanical machine you get an unpredictable new design that will be unpredictable unless it is tested to death. This is engineering 101, and the guys at Boeing who added MCAS to the Max knew it, the FAA guys knew it, and in the end the pilots who tested it to death proved it yet again.
Edmund