The 'quick and dirty' answer to that question is that the 737 MAX was a quick fix rather than a more expensive redesign from scratch. By adding bigger engines

The 'quick and dirty' answer to that question is that the 737 MAX was a quick fix rather than a more expensive redesign from scratch. By adding bigger engines and not moving the plane's center of gravity to the correct position by shifting the wings also, the plane becomes intrinsically unstable and no longer airworthy. That was 'fixed' by changing the aviation software to allow the computer to assist in flying the plane. That's all well and good, if the software is correct. But unfortunately for those passengers and crew concerned, that wasn't the case. They died. In colloquial terms, what they did at Boeing was 'put lipstick on a pig' rather than bear the cost of developing and producing a new, stable, airworthy plane.