I think it mostly depends on how much you put yourself out there and your personality. I've noticed that the extroverted popular kids jump from relationship to relationship, while the quiet and reserved ones have problems finding someone.
Maybe they didn't find what they are looking for. Those who find their lover easily it's highly common to break up very soon. Blind dates. It takes friendships and months to decide whether you match with this person or not.. well at least for me..