sdb;
One would have to consider the construction of both the plane and the structure. The plane of course is made from aluminum & other light materials, while the pentagon is built much like a fortress...very heavy, thick concrete walls. When the plane hit the building at what..around 500 mph.. the fuselage of the plane acted much like a piledriver with a couple hundered feet of material striking, and continuing to strike the wall..as a battering ram might do. The wings, even though of the same bacic material, did not have near so much continued battering behind them.
I believe if you view some of those photos, taken of the World Trade Center immediately after the strike, you will see wing imprints. At any level, but especially at that high level, the WTC was not constructed anywhere near as heavily as the Pentagon.