to lighten weight,not enough space,resources limited,needed them for another plane,put into production to early,ect. they have many reasons not to put FF(Forward Firing) guns.some stupid some smart.the B5N im not quite sure why.maybe they had enough escorts that they didnt need them.maybe the pilot was over-worked as it was

.ask Mitsu hell probable know...im more of a LuftWaffe,RAF,RCAF,USAF,USAAF historian.i know a little about the japanese planes but Mitsu knows way more then ill ever know.
to all