Gatsby expects that Daisy will be happy to leave Tom behind and break off her marriage with him. Gatsby's dream has been to build a fortune and reclaim the love that he once lost because, as he sees it, he was too poor for Daisy when they first met.
Gatsby's goals in the novel are defined almost exclusively in terms of Daisy. He confesses as much to Nick, letting Carraway in on the fact that he harbors what the reader comes to see as "romantic illusions about the power of money to buy respectability and the love of Daisy—the “golden girl” of his dreams".
The great wealth and glamour that Gatsby has attained do not prove to be enough to lure Daisy away from Tom. She too fosters a romantic dream, but the dream of being a certain kind of success, for Daisy, means that she has to stay with her husband.
She cannot do what Gatsby asks and say that her marriage has been meaningless, or, worse, a complete mistake.
Gatsby innocently, naïvely believes Daisy will denounce Tom and her marriage to return to him and the love they had experienced five years before.
Though Gatsby is correct is his assumption that Daisy still shares the old love that he feels, he is wrong to assume that she feels the same way about the malleability of history. She is unwilling or unable to undo her life in favor of a romantic vision of the future.