I think it might just have been a stark choice between (say) a P-G turret or H2S and that H2S was adjudged to be more important vis a vis bombing results etc,with H2S fitted it would have been very difficult to have any sort of underside protection without (say) having a slightly extended nose Lancaster with the H2S blister fitted under the nose (similar to the H2X/'Mickey' Blister under the chin of B17's).With the B17 on daylight formation raids of course not all a/c needed H2X as the formation bombed/'toggled' when the H2X a/c dropped the bombs on target during 'blind' bombing raids.