So far we got the following answers:

• Round cover is easier to transport (by nick_kt).
• Round cover can't go through if dropped(by alex).
• Round cover takes less metal to manufacture(by anon1).
• Round hole is easier to dig.
• Becasuse only circular covers can fit the hole perfectly in all the 360 degrees, where as other shapes will not do the same and leaves some gap on which if some one steps on will disturb its stability (by teegalapalli).