Why US submarines entered the Sea of Japan late

Why did the US Navy not send submarines into the Sea of Japan more aggressively before 1945?

Distant bases limited useful patrol time, and heavily mined entrances made access hazardous.

American submarines needed enough fuel to reach the patrol area, remain there, and return to base. A voyage that consumed nearly all their endurance in transit left little useful patrol time. Capturing islands and moving base facilities forward extended their reach, while reserve fuel covered contamination and other wastage. Heavily mined entrances created another obstacle to operations in the Sea of Japan.