Too slow - they got into the drag bucket and couldn't leave. AoA is high, so drag is high. Getting out requires more thrust, but they lost one and shut down the other on the fire warning. Even leaving the ground they started behind - a little too heavy and too much tailwind. If they drop the nose to get a better drag situation they lose altitude and crash. If they don't the plane just mushes along without enough thrust to get out of the bucket. Had there been no fire they might have lost enough fuel to climb out, but with the fire they weren't able to wait long enough.