So that if someone mistakenly drops it in on its side it won't fall into the hole. A square cover dropped mistakenly diagonally on its side would fall in.

Its because the round shape causes the impact of cars and trucks to be distributed evenly. If it was square, the impact would be highest at the corners causing the cover to break.

Tubes and pipes are cylindrical and easier to fit down a circular hole.

It's because a circular shape won't fall into itself. Manhole covers wouldn't do any good if it was at the bottom of the sewer.

Pot HOLES don't have covers. In some cities the pot holes are patched with similar shape fill as the hole. I'm guessing your question should read "Man Hole Covers" - but I'll just answer the question as posed ..

easier for worker to get down without hurting. also worker can just put a stick to hold up without fearing that stick will move to edge and fall down.

Well, some manholes (not potholes) are round, some are rectangular, but a good reason for them to be round is minimize the amount of material required while maximizing the girth of person who can fit into it or pipe that it covers, as pipes tend to be round.

Because pot holes are round

Was not sure of answer, but it is so that they dont fall in the hole