When the 747 first came out there were only two classes, F and Y. The "standard" configuration at that time was not to have any saleable seats on the upper deck but to arrange it as a dining lounge/bar etc for the F passengers. The main deck typically was laid out about F55 Y310. On the 747 Classic the upper area was much smaller than the longer type introduced on the 747-300; indeed the early 747s only had 3 windows on each side. The only reason for the "hump" initially on 747s was to put the flight deck over the nose cargo door that was in the original military freighter design. Many propeller military freighters had done the same thing.

There were some early 747s with charter companies that were Y class throughout; airlines such as Condor (Germany) and Wardair (Canada) operated them, with 3+3 seating up top.

There is a problem with having a seat class only available on the upper deck (eg all the F seats up there) that disabled travellers in that class would not be able to access it. This is why you need all classes to have some seats on the main deck.