My theory: The windows were designed to be less than a shoulder's width wide. This was because the architect had severe vertigo--an odd affliction for one who designs skyscrapers. I believe the people in the building were trying to lean out to get air. But in addition to the narrow width, the windows were floor to ceiling. Once the glass was broken, there was nothing to hang onto, except the wide beam that framed the window.
I have examined one of the jumper pictures with Photoshop. The man's lower arms are completely black. He simply would not have been able to hold onto the building with horribly burned hands. He lost his grip and FELL!
We must not assume that those people chose suicide, even faced with the horrific inferno behind them. Can't we at least give them that?
I would hardly call it suicide either way. They were already dead. It was simply their choice as to which way to die - burn or jump.