As has been suggested, bottom line is the constraints of a set trigger force less than ideal geometry when the trigger is used unset. The pin is forced to be low and through the plate. In addition, the distance from the pivot to the sear contact point is forced to be pretty long in order for the trigger to function well, when set. These factors result in a hard trigger pull. No "compound curve", (whatever that is), or perfectly designed / tuned lock can negate these factors. Just simple mechanics.
Jim