1. So they can’t fall in the hole if you tilt them the wrong way.
2. So you can roll them along the ground and move them more easily.
3. So the thermal expansion and contraction is more uniform and causes less stress on the cover.
4. A circular manhole cover requires less metal than a square manhole cover of the same diameter, so it’s cheaper.
5. No sharp corners to cause wear and tear or punctures to vehicle tires.
....and so that people that steal them can roll them right up in the truck bed..?? :)