BTS' album 'Journey' sells nearly 450,000 copies in Japan on release day

How did the Kpop group BTS become so popular internationally? I'm a bit behind and I don't understand how this group got so popular vs other Kpop groups. What makes them unique?

BTS - MAP OF THE SOUL: 7 - THE JOURNEY - Music

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BTS' album 'Journey' sells nearly 450,000 copies in Japan on release day

BTS - MAP OF THE SOUL: 7 - THE JOURNEY - Music

BTS Japanese album, 450,000 copies on the first day of release, New records of overseas artists

BTS To Release Japanese Album Map Of The Soul 7: The Journey

BTS To Release Japanese Album Map Of The Soul 7: The Journey

BTS's Japanese Album To Include A Song Composed By Jungkook, And More