They may simply have been born with a low sex-drive, as others
are born with poor eyesight or hearing.
More likely, it's environmental. Bad parental example. Sexual
molestation in childhood. Or in the case of deeply-religious girls,
they may remain hung-up on the idea of sex as a mortal sin. Or with
highly-academic girls, their intellectual development may have
overshadowed their emotional development, so they remained
unawakened until a man (hopefully) ignites them.