The guide said there are various rumors, but they're not exactly sure. The primary rumor is that the Navy fixed it that way when they gave it up so that it couldn't be used again. Of course, this seems dubious as it was an old ship in 1946 and no one was ever going to use it again anyhow. While it may've been fixed at that time, it also seems likely that a lack of maintaining the steering mechanisms in general could've contributed. There are some gaskets that have to be regularly changed out back there to keep the rudder turning, and no one has done any of that since it was handed over by the Navy. There was never any reason to do so and other maintenance tasks were always more pressing. There's a high likelihood all the mechanisms are frozen together at this point. It isn't leaking, so they aren't messing with it.
They are contemplating doing a t-shirt with the ship profile and the motto "Come and Straighten It", which I would buy if they did. Pretty good inside joke there.