Late reply, but still happy to answer.

Japan: former Imperial power that once even rivaled and threatened the United States, although decimated after the Second World War, the human capital and know-how remained, plus American occupation provided stability, during the Korean and Vietnamese war they were able to reindustrialize and expand their manufacturing sector by making weapons and getting aid from the Americans, plus Japan has a substantial population and working population

South Korea: colonized by Japan and had the life sucked out of it during the Imperial era culturally, economically, and politically, it was also a backwater that the US only intervened to protect from North Korea until very late in the KPA’s early advances (so the country suffered colonization and a civil war), afterwards the ROK was a dictatorship until 1987 and they had to start economically at a level that was poorer than most African nations, smaller population than Japan and they developed quickly albeit much later than Japan

Plus, Japan holds economic hegemony and old money since their electronic and appliance dominance in the late 20th century, however, Korea has caught up quite a bit.

TL;DR: Japan had a better starting point than South Korea after WW2, Japan developed earlier and the effects of that early market dominance still contributes to their massive GDP today while Korea has developed more recently and is still developing, plus a pretty big population difference