>>508536801 (OP)Your misunderstanding comes from not knowing the design features which made the twin towers unlike other buildings.
The old buildings (including the empire state building which had a plane hit it in 1945) were built using the steel frame method; where each floor is a series of interconnected boxes.
One of features advertised with the twin towers was their open floor design, which added millions of square feet of floor space; and was achieved by moving much of the support members to the outside walls.
Meanwhile, the method of construction of the wall sections used a series of sections which were welded together; with the floors attached via the industrial version of an L bracket.
Unlike the 1945 accident, when the jet hit, it knocked out such a large percent of the outside walls (especially at a corner), that it caused the floors above to first tilt, and then snap off (just like your image says should have happened.
As for the reason it collapsed... here we go back to the design difference. As the weight of the top floors started to fall, it hit each floor blowing out the brackets which attached both the floor and the wall sections.
The net effect (and if you look closely in slow motion you will see), that the building didn't collapse like a traditional building, but peeled like a banana