[Declanomous]

The new engine is more powerful, and at high angles of attack the nacelles also generate lift. The added power, along with the greater displacement from the center of lift/drag causes the plane to pitch up more easily, and the lift from the nacelle causes the controls to get lighter as you approach stall speed.

The FAA requires the controls to get heavier as you approach a stall. MCAS was primarily designed to address this flaw, which is why the added lift from the nacelles is an issue.