Why don't my parents let me go out with my boyfriend?!?!?

Ok, I'm 15, and I've been dating my bf for about 11 months now, yet my parents know him since he was a toddler, we grew up together.. I'm one of 3 sisters (other is 6, and the other is 17) and my bf is 16. When ever he asks me to go ANYWHERE, weather it be the movies, or just dinner with his family, they always say no. Ive tried talking to them, but they still don't let me. But, every time we ask him to go somewhere with us, he says yes, and to both of our accordance, it isn't fair.. I'm the most responsible of the 3, most respectful towards myself and the people around me, and they (my parents know that) but they still don't let me hang out with him. My mom is more "chilled" about it, and my dad is more strict about the situation...but still, i think almost a year of me dating him, and the fact that they've known him since he can crawl, is more than enough to let me hang out with him. I can't ask him to speak with my parents because it isn't his fault, nor responsibility, nor is it any of his business why they don't let me. Any suggestions? Anything you guys think I should do? HELP PLEASE! I need a very detailed explanation and the longer the better. Anything will be accepted !