Conspirators couldn't have known what the effect of an airplane hurtling into the towers would have on the wiring of the charges (link found via here). In fact, they couldn't be sure that the planes wouldn't stop the demolition process entirely, leaving a building or buildings standing filled with damning evidence. Controlled demolition is a difficult enough process without the wildcard of the effect of a plane crash. It is not possible that an intelligent conspirator would take that chance. This isn't even a debatable point.