The reason is because having a relationship at your age will be a distraction to your studies and it might affect your performance in school. Another reason is because you are not mature enough to make rational decisions in the relationship and you might end up being pregnant.
Maybe he feels your not ready to date. You could suggest that you will go out in a group instead of just the two of you or ask if he can come to your house and watch a movie. If he does come over tell your dad to stay in another room but this way he will also be able to supervise you. Have you tried talking to your dad about dating? Just ask him his reasons for saying not until your 18.
Im a father of 8 childen, 3 are teenaged girls, their are many reasons why, it would be hard to pin just one. But he may fear you'll grow to fast, or you get preg, or loose focus on you. Or he may not trust he himself as doing a great job at teaching you to responsible, or you may not be responsible enough. Greatest 1 of all......... scared of loosing his baby.
Thats what my parents say to. For me its because girls cost time and money. Its probably not the same reason for you, though. Maybe your parents dont wabt you getting pregnant to eraly. Or they want you to focus on acedemics. School always comes first.