During reentry, there is a 10 minute long window of maximum heating. They almost made it through all 10 minutes. Right at the end they lost their hydraulics. Makes me wonder if they could have flown the orbiter at a funny incoming angle to spare the load on the left wing. Maybe they wouldn't have gotten Columbia onto the ground, but if it had broken up five minutes later things might have gone a bit better.
There were 40 seconds after loss of control during which the Columbia pitched up into something like a flat spin, and the folks inside tried to get their hydraulic systems back.
After that, they had a depressurization that took less than 17 seconds and probably, hopefully knocked everyone unconscious. Nobody dropped their visors (which would let their suits handle pressurization). Apparently they were all in "fix the vehicle" mode and not in "survival as long as possible" mode.
After that the cabin seperated from the rest of the vehicle, the crew's shoulder and other restraints mostly didn't work, and they got thrashed to death: fatal trauma to their heads from the insides of their helmets. Owww.
From my reading, had they dropped their visors, gone to suit oxygen, and braced, several of the crew could have made it through both depressurization and cabin separation.
But then the cabin blew apart and they were in their suits in a mach 15 airstream. I didn't actually read this anywhere, but it sounds like most of the suits came off before they hit the ground.
Side note for camera geeks: notice how crappy the home video shots of the breakup look. Then look at the Apache Helicopter shots of the same thing, especially when it zooms in. That chopper has some nice telescopes!