Round is the only shape that no matter which way you turn it, it cannot fall into the hole. Square ones could be turned diagonally and it would easily fit into the hole. The same is true of triangular.

So it's a safety issue more than anything.


>This is a perfect example of why the question is silly and
>reveals nothing about the person you asked it to.
>
>Square or triangular manhole covers would not fall into their
>holes either (assuming they are placed correctly) because the rim
>of the hole supports the cover. This is not a function of the cover,
>its a function of the overall design.