Because controlled demolitions leave a ton of evidence and are extremely loud.    The explosives used in controlled demolitions must be loud if you want them to cut steel.  Keep them quiet and you reduce their power.   Demolition charges would have been heard in NJ and Brooklyn if they were used.   This is why people started looking beyond controlled demolition to things like thermite.  Except thermite also leaves tons of evidence.  It would have required tons of thermite to bring the buildings down which would have left tons of blobs of iron and slag.  Thermite is also insanely bright.  The whole building would have lit up. There's also no way a controlled demolition system (explosives or thermite) would be viable after being in a fire for an hour.  Controlled demoltions for buildings like this would take months to set up and you would need to be ripping offices apart because you need to get to the bare steel.  Buildings are also preweakened before the final demolition using acetylene torches that leave obvious fumes.  If you don't preweaken the buildings, you need stronger, louder explosives.  The dirt, fumes and noise would be noticed. Neither thermite or explosives would account for a building slowly losing structural integrity like was seen on 9/11.  In the Tower the floors were sagging and pulling in the outer columns.  In WTC 7 the corner was bulging out around the 10th floor.   A building slowly losing structural integrity is consistent with heated steel slowly losing strength.  In a  fire steel that heats up quick and then cools (like if that area of fire burns out)  will twist or warp.  The collapses were entirely consistent with fire.