Well the reason is the same for all three actually. Prior to the Sengoku period, most of the warring done in Japan was done by professional warriors who fought as horse mounted archers. Up close and personal fighting was incredibly rare and you need two hands to fire the bow, so that eliminates the Javelin and the shield. No body used the crossbow for the same reason; most warriors used bows. Guns turned up in 1543, making any crossbows they would have used essentially pointless.