There's a lot of stuff that has to break. Theory being that (at least until there's a certain amount of momentum) the structure of the undamaged building will slow the collapse. Remember, the damage was three quarters of the way up the building, not at the foundation.

I'm no demolitions expert nor am I a physics major, so I can't speak as to how structures of that size behave under those conditions - though I'm sure the conspiracies have been thoroughly debunked by now. But that's the thinking.

Personally I'm more inclined to disbelieve a conspiracy theory simply because there's no way our government is competent enough to keep a secret that big for eleven years - across multiple administrations. But who knows - maybe there're better at their jobs than I'm willing to give them credit for.