Mmh, let's try:
- so the cover does not fall in
- so that it can be put back easily
- so that it can be rolled on the ground (these things are heavy)
- to match the manhole structure, which is often a tube because that's more resistant to lateral pressure
- the size and shape are standard: new covers should match previously deployed manholes
- changing the shape would imply designing new foundry molds, which are expensive
- "we always did it that way" (general inertia will count as 0.28 reasons, completing the total)