Why aren't SR Motors used in all the heavy load or heavy duty applications ?
Switched reluctance motors require electronic circuitry to do the switching, so they don't have more than a century of history behind them like commutated DC motors, induction motors and wound-rotor synchronous motors. Preferences are always heavily influenced by experience. Also, the need for an electronic controller makes them less attractive for constant-speed operation in applications now served by induction motors.

You have not defined "heavy load" or "heavy duty." Electric motor torque and power ratings span about 8 orders of magnitude. Where in the range between cell-phone vibrator and steel mill does "heavy duty" fall?

The cost of SR motors will probably come down as manufacturing volume increases. However the cost of the motor and controller are not the only cost issues. There is also the cost of making the expected service life of the driven equipment as long as the expected life of the motor.

With volume manufacturing, SR motors can probably be competitive with induction motors in equipment cost, efficiency and all aspects of performance for applications requireing speed control. However, they may not be competitive over the entire power range of induction plus wound-field synchronous motors. It may only be a matter of time.

Update

A leading electric machine researcher has recently expressed serious doubts about the future of switched reluctance machines:

T.A. Lipo: Some Comments on the Present and Future Direction of Electrical Machine Research, Presentation to WEMPEC visitors, September 23, 2016