They didn't. 
That B-17F of the 332 squadron, 94 BG had its remaining elevator jammed in the nose down position. R/T chatter indicated that the pilots were going to try break it free. By the time they realized it wasn't going to move, they were nearly in a vertical dive at high speed. They had only about 1/2 minute to decide. No parachutes where seen before it broke up.
More than a few B-17s can home with a horizontal stab missing. As long as the elevator is free, it's flyable.
My regards,
Widewing