Why did my parents give up on me?

Well first off, while my mother was pregnant she used heroin and my dad smoked crack. I tested posative when i was born but with no defects other than my ribs and my breathing slightly, i have trouble with sports though, even though i won state for the 800 in track. But, my mother and father were very abusive... to the point i ran away when i was 5 but came home because i had no where to go.. im 15 now, i got away when i was 11 and live with my grandma. Everyday it bothers me, like y didnt they want me... and i have to be very careful on what i do because of the fact that i can get addicted to anything... please try to explain what i did wrong..