Just look at those nacelles. Deep breath. Look again. Take them in. Besides clearly visually screaming that this Frankenstein thing was quickly&cheaply slapped together and wasn't properly engineered and thus should just have never seen the light of the day, these nacelles obviously add more lift than normal symmetrical ones. So:
1. the engines placed more forward than pre-MAX 737 - that results in additional pitching up moment as the engines are below the centers of pressure, gravity, etc.
2. the engines are 2x higher-by-pass than pre-MAX 737 and thus the center of thrust is shifted even more forward and lower - as a result it adds even more of the pitching up
3. these asymmetrical nacelles generate more lift just due to the shape - and again due to the position of the engines that lift results in the additional pitching up moment.
Basically that thing just can't really fly steady straight, and looking at all this some people at Boeing decided that a bandaid software patch would just fix it. Sounds like it were the same people who did the "curl" fix in today Cisco story https://news.ycombinator.com/item?id=19508472 :)