because the pentagon was built to withstand bombardment from the air. THe WTC was designed to withstand a hit by a jetliner ( and it DID ) the combination of the impact damage and fire was tooo much. the WTC was a radical new design. NOT traditional "steel frame" construction. very very light for its size. traditional "steel frame" method would have made it too heavy to be self supporting and made for too little rentable space to make the project economically feasible.