Because of the bad reputation the house has gotten, with most deatheaters, otherwise known as Voldemort supporters, hailing from the Slytherin house. He mostly doesn't want to be apart of the house because of Draco Malfoy. Draco Malfoy seems stuck up and rude, and Harry doesn't want to end up like him.
Hope this helped!
Your friend in answering, The Slytherin,
~Steve
