Very few girls (or boys for that matter) can succeed outside pop. Pop is after all where most of the audience and therefore money and fame are. Those who do become big in other genres either come from a famous musical family (Cleopatra Stratan, and even her has become popper; Kelly Osbourne, Ángela Aguilar... and even there, Miley Cyrus always sang pop despite coming from a country music family) or enter niche genres with relatively little competition (Taylor Swift began in country; Wendy Sulca in Andean folk; Janeth Guadalupe in Mexican cumbia, Prussian Blue only ever played neo Nazi music...)