Their crimes were not simple disobedience or the breaking of a few minor rules. They perverted the entire order of all things, they put all of Creation in danger with their actions, they tried to ruin the Creator's plan; thinking they knew better. Their bodies were taken and they were left but mere 'thinking breaths' upon the wind. Breaths which could enter the forms of human beings and control them. Exhalations with a plan and the infinite patience of those who really do have all the time in the world...there is the irony for they seek nothing but the end of that world.
The Fallen were denied their bodies when Michael delivered the Judgement of The Creator upon them in Atlantis just prior to the beginning of the Flood. They were exiled to Earth until the Last Day but denied their true bodies. Instead they learned to possess or ride the bodies of human beings. Those who are willing allow the Fallen to ride them for the rest of their lives, there are always benefits to those who make such deals. They are always corrupt or godless individuals, that is why the Fallen chooses them. At times such a "mount" may dies before a Fallen has time to select a new one and he must find a vessel, someone to simply transport his spirit until he can get to where he needs. Such vessels are not taken over by the Fallen but the mere presence of the alien spirit will cause at least nightmares and, usually, vivid insanity. It has been known - most notably in the case of Legion who was a vessel for up to one thousand such spirits - for multiple 'homeless' demons to hitch a ride inside of one body at the same time, though this is rare.