Apparently the 737 MAX (without MCAS) does not have acceptable pitch characteristics with flaps UP at airspeeds approaching the stall. Therefore ...
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during ... flaps up flight at airspeeds approaching the stall.