Because if it's one thing I learned from actually being in Korea, it's that international and korean views on things are entirely different.
BTS IS very popular, just not so much in their own country. The average person in korea is not really aware of them honestly and they don't have a huge Korean fanbase. Their fans are mostly young and really dedicated. However, they are really well liked musically amongst kpop fans there in general. Even among people who aren't bts fans.