It's been edited! The part where the tower is actually collapsing has been edited out, between 4:03 and 4:04. The camera has just zoomed in, then zooms out, then at 4:04 someone says, "Ah!" but it's like his comment has been cut off before that. Then someone says "Holy crap, the whole side, it's gone, the whole tower!" But there is no reason why they wouldn't have seen and taped the actual collapse.
This is the real enigma. Not the fact that the tape was never release, but what happened to the collapse footage.