Genetics, he inherited it from his mother, Banchina. Althouhg it seems as a female, her nose wasn't as long but it still was longer than the norm. theres a pic Some will say that Oda drew Usopp to have a long nose as an allergory to Pinochino but there are many charaters with long noses in One Piece = Kaku, the Square Sisters, Porche-chan, Foxy are some