Engines above the wings?
Simplifying the "structural" consideration: All of the lift comes from the wings. A lot of the overall weight is the fuel, some more of the weight is the engines. Put fuel and engines where the lift is means that you don't have to work out great bits of metal to transfer the lift to some of the heavy bits!
But why have the engines slung under the wings, when putting them on top would presumably reduce FOD problems as well as increase ground clearance? (B wouldn't have had to have that odd oval engine casing on the 737). Only reason that springs to my mind is that TOGA power would result in a pitch-down moment, as opposed to pitch-up when the engines are under the C of G - probably not desirable!