But really, it was just about four-engine planes becoming too expensive to run. Two-engine planes won. 777 burns 30% less fuel per passenger and has almost the same cabin width. And top level became a flop because it's too narrow for a first class cabin by today's standards and all other uses for them make no sense. Top floor existed at all because it was Boeing's entry for a heavy cargo plane competition in which C-5 Galaxy won: it was meant to be a cargo plane with a small - top floor - passenger cabin.