The force generated from the 100.000 ton tower falling is greater than the resistance from the floors below. As the tower is falling, energy is generated. It gains momentum as long as it keeps moving downward. Even though it loses some energy hitting the floor below it, by such a heavy object falling it regains that energy, plus some extra energy, and hits the next floor even harder than it did the floor above it.

If you remove the top 30 floors from the twin towers, where it now has 80 floors, and drop an 100.000 ton anvil on top of the tower, floor 80 will be crushed, as it is not designed to withstand an 100.000 object falling on top of it. No part of the building was designed to have a 100.000 ton object, that be the top 30 floor of the tower, drop on top of it, which is what happened.