In the past, boxers would fight in a circular area, sometimes with a rope around the edge, resembling a ring. The name has stuck ever since, even when the standard shape for a fighting zone became square. As boxing evolved and rules began to be standardised, ropes around the sides of the ring were introduced, and the easiest way to do this would be with four corner posts. So the fight area has been a square and sometimes is referred to as a ‘squared circle’ in reference to the origins.
Boxing Unboxed 