Could be a lot of reasons.
-She made the already existing R&B style into a popular style, more american sounding in Japanese.
-She was pretty young and wrote a love song like First Love>
->First Love which got really well marketed using a popular drama that fitted to it, that way a lot of people could get to know it.
-A lot of people in the process probably liked that she is young and writes and composes everything herself.
-I don't know if it was known then but maybe also because she is the daughter if keiko utada (her mother) that was a famous singer in the 70s or 80s in japan. people got interested in utada the daughter.
-Since Automatic she already got good tie-ins, it was a variety show's ending theme, Movin' on without you was in Nissan Terano car CM, that's probably how a lot of people started to discover her from.
-Using First Love is the album title/theme was a smashing great tactic.
She might be awesome but that's not a reason for a phenomenal success
