I remember reading about the development of high-end market (as in # of passengers) back when development started. I had a subscription to Aviation Week back then, and similar to what Skybird said, Airbus was basically building an airplane on specifications and assuming/hoping the airline market would move in that direction.
Boeing decided to design planes to fit the market as it existed at the time without trying to project what the market could possibly be. With larger/more efficient derivatives of the 747, Boeing already had a plane that could compete fairly closely in the super-heavy market, and they shut the whole line down because they didn't think there was a super-heavy market.
In the 70's-80's, the 747 was a PR goldmine for Boeing. To many, it symbolized the glamour or airline travel for a long time. People talked proudly if they had ever flown on a 747, and personally, I think at least part of the reason that the A380 was built was purely for bragging rights to have a "bigger" plane than Boeing produced. Unfortunately, the airline industry moved on from glamorous travel and fully into mass transit in the meantime, and super-heavies don't work well in that market for the same reason you don't have city buses with 200 seats. You can't fill them.
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