The Manhole covers are round because they are extremely heavy. A circular cover cannot be turned in any orientation that would allow it to drop through the hole it is covering. The fall could be a great distance and potentially break stuff or kill someone working down below. A square cover can be turned at a 45 degree angle from its diameter and dropped through its own hole.
Additionally round covers can easily be rolled out of the way after being lifted off, versus being needed to carry.