more than anything, it probably had to do with space. in the tail, the gunner had to lay on his belly(in the b17). even in the lanc where he didn't, there's still a lot less room back there, than there is in the front of the aircraft.
i had read a book called "combat crew". it was pretty good actually. it seemed that quite often, the gunners would sneak extra ammo on board if they found out that they were going to a particularily heavily defended target, or if they were going deep into enemy territory.
separate note.....i think i've never been ho'd in bombers. mostly from my 6. sometimes six low, where they just park there, and let me shoot at a stationary target.
when i see a guy up above me, taking his time to set up for a nice pass, then i worry a bit more.