The towers were not built well, on trusses with a giant truss over the Con Ed station. It had no central supporting columns. Its main support was the outer skin. Once that was breached, the building lost integrity. Notice how the building came down top to bottom, not bottom to top, as is the case when a buidling is taken down.
As for WTC 7, it was brought down because the structure was too weak from the explosions next door. Windows can blow out from explosions 6 miles away, so just imagine next door.