Why is motor back emf constant? Motor torque is dependent upon armature position which takes into account position of electric fields. Thus if you have a DC motor with a commuter you get positions with basically no torque (just prior to switching current direction).
My question is, why isn't back emf also dependent on motor position?
 A: The back emf is constant for a given rpm.  With increasing load the rpm decreases and the back emf decreases.  AC motors are different; for example a synchronous AC motor has constant speed while an induction AC motor slips (reduces speed) with load.
As far as rotor conductor (coil) positions are concerned, for a simple "cartoon" DC motor discussed in elementary physics tests, the magnetic field is constant in one direction and the back emf in a conductor on the rotor is a chopped sine wave as it rotates as @niels nielson states.  This sine wave has amplitude and period determined by the rotational speed of the rotor, both constant at a given rotational speed.  The average emf (sine wave over time) decreases with decreasing rotational speed, and the speed decreases with the load on the motor.  So, for the cartoon DC motor the instantaneous emf in a coil does depend on the rotational position of the coil, but over time the emf sine wave characteristics and the average emf depend on the rotational speed and not the coil position.
For an actual DC motor, the magnetic poles (field shape) and conductors are constructed such that the emf generated by each coil on the rotor rotating at constant speed is essentially constant except for the brief time when the conductor is between the poles (where the emf is zero). The total emf is that of all the conductors, and depends on the speed of rotation of the rotor. As @Jun Seo-He suggests, you should consult the EE SE and/or an electrical engineering text on the design of electrical machinery for the details of actual motors.
A: It is in fact dependent on rotor position in instantaneous terms (it traces a sine wave or a chopped sine wave as the rotor revolves) and when the motor is spinning without load at its maximum RPM, the instantaneous sum of the supply voltage (which varies in time as the commutator opens and closes the coil circuits) and the time-varying back EMF is zero, or nearly so.
