There are many reasons. Each of them is described below.
- If covers are square or rectangle then there are chances to accumulate fungi or algae or other sewage material at corners. Since circular pipes have no edges this could avoid the problem
- Round covers are easier to manufacture than other ones
- The lacing of cover on manhole is easy as against of rectangular. Say, a rectangular cover has chances to fall into the sewage pipeline.
- Easy to place and manufacture in bulk.