Why were the allies not afraid of Japanese submarine and afraid of German submarines
The problem with turning Japanese submarines loose against the sea lanes was
Because the Japanese used their submarines as auxiliaries to the fleet, not as commerce raiders. It was the way they were used not the quality of the boats or crews that made the Uboats such a threat.
Barry has already said it but I think it's more than IJN simply doesn't have a target to cut off that can be vital like KM doing so to GB mainland? Cutting of PI was already done by surface fleet, and they can't cut the continental US.... a sub campaign against US simply can't be decisive and Japan needs a short war anyway.Because the Germans knew how to use their submarines