The Japanese had been fighting land wars, while the Koreans had been at peace on land but fighting pirates at sea. So by the time the war started the Japanese had not developed ships capable of mounting cannon, and had instead invested in lighter, faster vessels adapted to grappling tactics and, ironically, muskets and small arms. (I've seen claims that Japan only had softwood for ship timbers that was unsuitable for handling cannon recoil, but that may be an exaggeration.)