The square ring allowed for better viewing and safe distances.
According to The Encyclopedia of Boxing, by Gilbert Odd, the term "boxing ring" goes back to bare-knuckle days.
Spectators held a circle of ropes around the fighters, but they were always crowding in on the action. Eventually, they used stakes to hold the ropes, forming a square.
What Maurice said
What Hex said!
because it confuses the boxer when they have to go back to their corner if the ring is round.........
what LuvableDog said~