from alex:Round cover can't go through if dropped.
from anon1:Round cover takes less metal to manufacture.
from :Because round holes are easier to dig.
from teegalapalli: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.
from Alan_App:Thermal expansion will be symmetric for the circlular
manhole,explosions? plus it's gotta make life easier for
sewage inspectors...I imagine there is quite a lot of
weight in those things.