Simply because they didn't have as big an impact as the Germans when it came to submarine warfare.

The German and Japanese navies operated with entirely different submarine doctrines. The IJN saw their submarines as naval vessels meant to help the Combined Fleet and attack enemy navies. During the war, IJN subs were rarely sent to attack Allied merchant shipping, or to cut off allied trade. Instead, they were operated in support of Japanese surface vessels (such as I-168 operating to help the Midway invasion force and sinking the crippled Yorktown), against Allied naval forces (such as attacking the Okinawa and Iwo Jima invasion fleets), as resupply vessels for isolated Japanese-held islands, and for special missions. These special missions included trade trips to German-held France, shell attacks on ports such as those in Australia, and the deployment of midget submarines (Pearl Harbor). Merchant vessels were not the primary targets of IJN submarines all the way through the end of the war.