Why was Harry Potter adopted?

Harry Potter was not adopted in the traditional sense; rather, he was placed with the Dursleys, his aunt and uncle, after the death of his parents, James and Lily Potter. Following their murder by Voldemort, Dumbledore left Harry with the Dursleys to protect him from the dark wizard and to ensure he could access his mother's magical protection. The Dursleys were his only living relatives, but they treated him poorly, unaware of his true importance in the wizarding world.