A torque-speed curve like the one you show depends on both the motor and controller, as has been discussed here in the usual congenial manner with regard to AC versus DC motors. For a series DC motor that "flat" part of the curve to the left is where the controller can put out enough voltage to push it's max current through the motor, which gives max torque. The fall off to the right is where the current through the armature is decreasing due to back emf of the motor increasing with speed. Back emf is also of course increasing with speed at the lower rpms to the left, but the controller can increase voltage to compensate (limited by battery pack voltage). If you want to extend the flat part out to higher speed, you need a higher voltage controller and/or a higher voltage battery pack that will give the same max current. More max torque requires higher max current.
Maybe major can add details on how motor design affects this.
Nice explanation on motor armatures major!
