Manhole covers are usually made in circular shape primarily to avoid them from falling into the hole they are intended to cover. But, there are two other reasons: circular shapes are slightly easy to die-cast, plus, manholes with circular shaped covers are very easy to "close" (the cover has no specific position, so the operator just needs to worry about centering it).

To provide a little more details about the primary reason for the circular shape of manhole covers, imagine any other shape. For instance, an squared cover of side L. The whole to cover will need to be also squared, so the diagonal of it will be approximately \(L\cdot\sqrt{2}\), which is bigger than L (the distance between opposed edges). Meaning that the cover can fall inside it. And it will happen the same to any other polygonal shapes you might think of.

But, in the case of rounded covers, the distance between opposed "edges" will be its diameter. And it will sit perfectly on top of the hole to seal (as they will have a small lip, reducing its diameter a bit below the cover diameter). Therefore, there are no chances for the cover to to fall inside.