TBM - almost Beaufighters carried 4 cannon and 6x 0.303 -
Same reasoning as given by JTO - you might only hit the enemy with a few rounds and they had better do damage - W European military aircraft were generally better protected than either US or Japanese aircraft - certainly at the start of war .
Several US fighters imported into Europe early on showed substantial performance loss due tot eh need to fit armour, armoured glass and self sealing tanks....... the original P-51 being a case in point.........................