Why are some people not interested in sex and intimacy?

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.