Why doesn't this girl like Romeo?

Who knows? Since Rosaline doesn't actually speak in the play, we do not get a good idea of her character. We are told that she has "sworn to remain chaste" but that was a pat answer to an unwelcome wooer at the time, so it tells us nothing. Does she have another guy? Does she not like Romeo's type? Does she hate Montagues on principle? Is she a lesbian? Does she plan to become a nun? Is she already secretly married to someone (the friar appears to be willing to do this). There is no way of telling.