The round vault doors were probably easier to make, in that size and at that time, than any other shape. Round is indeed simple and reliable, I am not aware of any successful breakin through such a door. Usually a vault is compromised by tunneling underneath (e.g. Baker Street robbery - Wikipedia, the free encyclopedia or Albert Spaggiari - Wikipedia, the free encyclopedia), or occasionally by extreme cleverness (such as The Untold Story of the World’s Biggest Diamond Heist).
More modern manufacturing allows rectangular openings and floor-level sills, and electric/electronic alarms (actually, the rapid response they enabled) removed the need for such massive doors.
That patent is interesting because it discusses one way to construct the hinges. One of many patents that describe the door itself is 816,049. I have a small pile of similar patents if anybody really wants to build a model!