Probably wasn't hard. I know this area was also an issue with 71/72 cars as well. Since it's located on the top of the car it didn't have the attention to rust prevention the underside got, and I wouldn't be surprised if there wasn't some areas of the void left unpainted during manufacturing, which would cause rust to form very quickly. Whatever happened it was significant enough to warrant paying for an engineering change for a part that was only going to be used for one production year, and Chrysler never liked doing that.
I've also wondered why they changed the trunk floor design between 73/74 and put that hump back there. I think someone told me why they did that years ago, but I can't remember what the reasoning was.
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There's actually quite a few differences. The hood was changed. The front suspension was redesigned. 1974 was the first year for retractable shoulder harnesses, so the interior design was changed to accommodate mounting points for the harness swivels and the retractors, and the quarter panel covers were redesigned into two-piece units instead of a one-piece. 74s also have a hump in the floor of the trunk that 73s don't have, and I'll be danged if I can remember what that hump was intended for. 74 was also supposed to be the first year for the system that prevented the engine from being started without the seat belts being fastened, and had an odd little box under the hood with a button on it that worked as a temporary bypass for mechanics to start the engine. I say supposed to be because I had a 73 road runner that had that system installed even though they weren't supposed to have it.
