Why does an aircraft have to be flying straight and level to facilitate a jump?
One of the reasons why many WW2 pilots did not make it out were the G-forces and acceleration in directions other than the door when they had to get out. The bailing out survival rate appears to be correlated to the ease of exit. 

If you're going to have a smooth and safe departure, you're happiest doing it during level flight when there's no interference from plane motion.


(source: lonesentry.com)