I believe that mostly stemmed from the B-24's ball turret, which actually had to retract into the fuselage during takeoff and landing to avoid hitting it.
Also think about the construction. The Lancasters (among many other RAF bombers) used a honeycomb airframe which would enable the aircraft itself to be VERY hard to down, but the crew members themselves were relatively unprotected barring some 'armored' glass, contrary to this is the B-17, which...