Gatsby tells Nick that his parents are dead because he does not want Nick or anyone else to know anything about his origins. He is clearly not from a wealthy family or background, and throughout the entire novel, he does everything he can to obscure his beginnings. Remember that his mission is to win Daisy, who is from a wealthy family, old wealth, not nouveau riche, as he is, and he believes that his real background would not appeal to her. In spite of how much he loves Daisy, Gatsby realizes deep down that she is a material girl. Think of how impressed she is by his just all of his shirts! Whatever Tom Buchanan's failing are, and they are many, he is from the same world as Daisy, and Gatsby believes he must be able to compete with Tom. Gatsby relies on concealing his origins and on the wealth he has managed to accumulate. But if Daisy knew that he was from a lower-class background, he is sure he would be out of the running. And as it turns out, he is correct, since Daisy will not leave Tom for Gatsby. She knows that Gatsby is not from her world, no matter how much money he has, and thus Gatsby is unable to attain his dream girl.