Well, there's no real way to explain it besides that teenagers are shallow. Don't think that it's just your weight though. If your personality is good, but just shy in talking to boys, that could be your downfall. Remember that most boys DO NOT want to make the first move. If I may talk about myself for a moment, I'm very skinny, and I think of myself as attractive, but I don't get into any relationships because I'm shy. Even if you aren't shy, you might want to talk to more guys, and be open for more relationships that you didn't see before. You'd never know that some of the best guys out there like you, but they just don't want to admit it.
If you really think that this is your main problem, then do something about it! Try working out, and losing a few pounds. See what the girls in relationships are doing differently than you are, but don't change yourself, work with what you've got!
Above all, teenagers are shallow. I'll say that about guys AND girls, because it's all in our hormones. If you can't get the best, then you're going to have to settle for second best until you CAN get the best. The teenage years are often the hardest ones, and you'll just have to live through it. Almost all of us go through times where we just can't get into a relationship, but we've gotten through it. I'm sure that you will too.
-------
Take me above your light
Carry me through the night
Hold me secure in flight
Sing me to sleep tonight