I believe it was due to the shift in the center of lift to the center of gravity. Maybe that was caused by the larger diameter engines.
By my admittedly superficial understanding, there is a center of gravity aspect to the overall aircraft design problem, but MCAS was apparently aimed at counteracting some abnormal (?) lift at high angles of attack (climb rate) caused by the engine positions that could affect the feel through an unassisted yolk (reducing "stick force" and confusing the pilot), causing the pilot to pull back the yolk more than necessary, and possibly putting the plane into a stall situation during the crucial climb-out phase. That's in the link I provided.
I have no expertise in this field at all, but I do remember the first time I tested a feature called lane-keeping assist in a new Mercedes I test drove. Perhaps I'm being a big baby about stuff like that, but when I purposely tried to drive over a double yellow line it was very disconcerting to have the vehicle's electronics take control of the steering wheel. I suppose I could get used to it, but it would take repeated practice. I can only imagine how some pilots felt having the yokes in a jetliner becoming unresponsive and the plane executing non-commanded actions without prior training.
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