Clam shells will also work. They have a high clacium content. Just crush them into small enough pieces so the chickens can handle them. You can also use lobster shells, crawfish shells, crab shells, whatever you have handy. Chickens also get calcium from hard-shelled bugs. That's one reason free-range chickens seldom need a calcium supplement.
Oyster shells are used since they are abundant, hence cheap. You can take a dredge out to a dead oyster reef and fill up several barges easily. The oysters in a live oyster reef are too valuable for them to be dredged. Oyster shell dredging is banned in certain environmentally sensitive areas, like Lake Pontchartrain.
I could go on, but the bottom line is that oyster shells are used because they are cheaper and easier to get than other shells.