This may be a stupid question and with all due respect,
If they were skydivers, why didn't they put on their parachutes and jump?
Easy answer - because it was probably impossible.
I've had to bail out from a damaged aircraft only once, at an event in Illinois. During jump run (i.e. when everyone was preparing to jump) someone's parachute deployed in the door. Parachute went over the tail, he went under. It felt like the plane had hit a wall; it suddenly pitched down 30 degrees and slowed dramatically.
Now, in this situation, everyone had their parachutes on and was ready to exit. The door was open. I yelled "parachute over the tail! Bail out!" so they knew what was going on. It _still_ took 30 seconds or so to get everyone to understand what was happening, and get them up the slope to the door (which was in the back.) And this was with skydivers ready to exit; the only oddity was a slope they had to climb up.
Now translate that to what likely happened here. A plane was packed with jumpers and their gear - and if anyone has flown in a Caravan, they know that 10 people in that plane is a little cozy. It was dark out, and for at least part of the flight they were in the clouds. At some point the plane began to spiral, which means G's. The odds of finding your gear, in the dark, and getting it on with 8 other people trying to do the same, while pulling 2 G's are pretty low.
Also keep in mind that the pilot was likely still trying to fly his way out of the problem, and would have probably started yelling at anyone who tried to open the door while he was trying to solve the problem of being in IMC at night over mountains.
There have been a few cases like this where people HAVE bailed out, but it's generally a close thing. In one case, a pilot had the tail torn off his Cessna 206. He had his parachute on, the door was open, it was right next to him and he knew exactly what was happening. He _still_ barely made it out in time; he exited the plane at 1000 feet, which is about five seconds from impact (and about two seconds from being too late to open a parachute.)
So generally unless you are ready to go and are expecting to exit, the odds of being able to use that parachute in an aircraft emergency are pretty low.