There was a time when J-pop used to be popular along with other products of the Japanese entertainment industry such as J-dramas and Japanese variety shows. Big names such as Utada Hikaru, Ayumi Hamasaki and Namie Amuro are quite popular, especially in Asian countries. Nowadays, it seems like the Korean wave, or hallyu, has captured the attention of fans worldwide. Based on the opinions I have gathered from my friends, one of the reasons many of them prefer Korean idol groups more than Japanese idol groups is because they find the Korean group members more attractive.
This is because Japanese groups, even popular idol groups like AKB48 and Morning Musume tend to not select only good-looking members with perfect features. Instead, many average-looking members are chosen and then groomed to improve over time. I think they prefer average looks which you will find more endearing the more you look at them instead of a stunning beauty which you will be less impressed by over time.
The Japanese seem to appreciate unique beauty and imperfections so idols with features which are not considered universally attractive such as crooked teeth, hooked noses and short stature can also be wildly popular or be considered beautiful. It is also not always the case that the most popular members are the best-looking ones. Hence, Japanese idols seem more down-to-earth and appear to be more relatable to regular people.
This is in contrast with Korean groups’ tendency towards perfection. Each member tends to be tall (or at least made to look tall with very high heels), be good looking with near-perfect facial features, and be equipped with singing and dancing abilities from years of training. Usually, the most attractive members will be the most popular. Thus, in a way, Korean idols are like the untouchable gods and goddesses shrouded by a veil of perfection with standards deemed unattainable by regular people.
I think that the Korean style of perfection tend to capture people’s attention easily which is why they can be popular worldwide in a short time. In contrast, the girl-next-door or boy-next-door beauty of many Japanese idols tend to only be appreciated upon seeing them many times. It’s a matter of “acquired taste” beauty versus beauty which immediately attracts you!