Boeing took a design that was not intended to run active fly by wire systems and then added flight dynamic control software (MCAS) and perhaps other stuff onto an avionics infrastructure platform that was never designed to cope with mission critical flight control. There needed to be three systems, voting, separate busses, permanent magnet generators, etc. etc. to implement MCAS as a mission critical system.
To put that another way, 737 Max was a hot rod, not a production car. It looked like a 737, but it was a kludge.