Adele too and arguably Taylor.
I think the difference that they don't make music that most people think is 'real music'. Idols like IU and BTS are known to make music by themselves(which 90% of KPOP can't do or aren't allowed too). The same reason why people like Ariana and Justin aren't really appreciated outside of the teen/20s demographic compared to people like Taylor/Adele/Ed, they can't really write alot of their songs(Ariana has improved in this regard though) and their music is considered pop which isn't really a genre for respected artists.
Ed/Taylor/Adele etc most of them are known for their songwriting but for the music their music is more catchy and have very simple chord progressions esp songs of Ed Sheeran, that's why its more memorable for everyone i think. Also Adele, Taylor & Ed look modest & mature to most people thats why they might be interested.
for kpop, yeah the one reason where people dont take kpop seriously is they think idols dont make music by themselves and they think their music is not that deep and dont cover serious topics. From groups i stan SKZ write & produce all their music and they cover topics related to youth & teens. Maybe that's why older people dont relate to their music. But BTS's message is more universal "Love Yourself" which is directed to everyone. For the music part it's hard for kpop to become mainstream & go outside teen demograph since its very experimental, intensive, past faced, unconventional. Older people might prefer listening ballads, R&B etc much more slow paced & catchy music.