Maybe because cuboids are harder to open with a can opener? All the cuboid cans I've seen have tabs on top and thus don't require can openers. It's conceivable that tab-opened cans are more expensive, don't preserve food as well, or something like that.
Sure, but it still requires the same kind of prescored lid; even with a key, I don't think that general design approach scales up very far while still not being too much of a hassle for consumers to bother with if there are competing products that are more convenient to open.
