There are two reasons;
1. The planes hit the towers at high speeds lowering the structural integrity of the building.
2. The jet fuel caught fire and caused an explosion in the tower itself which in turn caught fire.
It was not just one reason.
In the surrounding area, there were cars which had their paint and metal scorched. However certain materials were intact, such as the sirens on the police cars, which would melt at a lower temperature then metal.