weakened the steel enough to initiate the pancake collapse why would explosives need to be placed throughout the building to produce the same result?

If “pancaking” initiated by weakened steel in the general area of the impact is a plausible hypothesis then theoretically, a demolition hypothesis where explosives are placed only in the area of impact would be plausible as well.

Why would a 100 tons of explosives be required?

does the force of gravity weaken w/o fire or something?