Because of cloud coverage and a smoky atmosphere due to prior day bombing runs on Yawata. A fuel pump was also deficient, thereby preventing the crew from using the fuel in the reserve tanks. Carrying all that fuel and fatman back would have caused them to run out of fuel before getting back to base. Since Nagasaki was already a secondary target, and a cooperative hole in the cloud cover appeared over Nagasaki, voila, the crew was saved from ditching in the sea, and possibly even drowning. I still wonder what would have happened if Nagasaki was also heavily clouded and they were unable to drop the bomb in Japan. Would they have dropped it elsewhere just to get back home? We will never know.