Because Israeli art students set detonators onto the explosives that were already pre-built onto the main support beams of twin towers. Those explosive were placed there in secret, during construction by the tower’s builder David Rockefeller. The excuse used for the placement of explosives during construction was that it would save money on the eventual demolition of those buildings. Without detonators, those explosives would stay inert for as many years as necessary. However, David Rockefeller may have know what the preset charges were really for.