Incomplete pass ruling

In sunday's Chiefs and Bronco's game, the Chiefs qb was being tackled. His upper arm wrapped up by the defender. His arm, from elbow move forward and the ball was released. Ruling on the field was incomplete pass. My question is: If it was an incomplete pass, why was there not a penalty for intentional grounding? The qb was in the "box". the ball didn't get back to the line of scrimmage and there was no receiver in the area.