extra software is required because the forward placement and height of the LEAP engines imparts a moment which can cause the plane to pitch up, putting it at risk of aerodynamic stall. boeing put in a new stab control routine that would automatically pitch the nose down in case the AoA became too high, so that the MAX “felt” the same to pilots as the prior 737s with CFM56 engines. this, because they didn’t want to have to get the airframe recertified but sucessfully lobbied the FAA to grandfather the MAX on the prior 737NG certification.
then, they didn’t tell anyone about that new stab control routine. 737 pilots are all trained for runaway stab trim, but since this system would kick in intermittently, each time incrementally moving the stabilizers up, pilots were not likely to recognize the situation as a runaway stab trim situation, where the trim wheels would move continuously. therefore they never thought to reach for the stab trim breakers. and 2 planes crashed.
