Because due to the different shape and position of the nacelles, there are aerodynamic effects that cause the plane to want to pitch up at high AoA. If you are already close to a stall condition, and you were caught unawares, bad times would be had.

The major issue though is pilots were left to 'just figure this out' instead of being informed of the change. I have a feeling Boeing felt justified doing this because they pitched it as a "reconfiguration" of an old airframe, not warranting any explanation.

As with any task I've attempted that involves deadly failure modes, this should have struck someone as being a terrible idea.