A round cover can't fall through its own hole, because its width is the same in every direction
A square cover rotated diagonally could slip through the opening and fall on workers below. A circle's width (its diameter) is constant no matter how you turn it, so it can't drop through. Circles aren't strictly the only shape with this trait — any curve of constant width works, the Reuleaux triangle being the classic example — but a circle is the simplest to make, which is why it's the standard answer to this famous engineering interview question.
