Maybe they have something in their musics like 'wow' factors. One more things, their music is so addictive, it's like if you heard once you wanted to hear more, their discographies never get boring, there's a lot of different type and you can choose according to your moods, they talked and covered about many issues.
I'm not sure what, but each of BTS members have their owns unique vocal colour which might be lacking by some group, cmiiw.