What quote in Chapter 8 explains why Daisy married Tom instead of waiting for Gatsby?

In Chapter 8, Gatsby explains to Nick (who relays to the reader) the story of how he met and then lost Daisy.

According to Gatsby, he was penniless when he and Daisy fell for one another, but he acted as if he was rich like her and would be able to take care of her:

"...he had deliberately given Daisy a sense of security; he let her believe that he was a person from much the same stratum as herself—that he was fully able to take care of her."

So we assume that Daisy didn't stop waiting for him because of money, since she was led to believe he was rich.

While he was away at war, Gatsby stayed in touch with Daisy, but in her letters she seemed to be getting impatient waiting for him to come back: "—there was a quality of nervous despair in Daisy’s letters. She didn’t see why he couldn’t come." This shows that Daisy loved Gatsby, but she was too impatient to wait an uncertain length of time for him to return and marry her. Gatsby was away at war and nobody knew when the war would end, or who would survive.

Daisy was immature and materialistic, living in a world of money, fashion, and flowers, which throughout the novel symbolize the short-lived beauty of love:

"Through this twilight universe Daisy began to move again with the season; suddenly she was again keeping half a dozen dates a day with half a dozen men and drowsing asleep at dawn with the beads and chiffon of an evening dress tangled among dying orchids on the floor beside her bed."

The beads and chiffon symbolize wealth and parties, and the dying orchid symbolizes Daisy's dying love for Gatsby. Daisy was also impatient and decided not to wait any longer for Gatsby to return:

”She wanted her life shaped now, immediately—and the decision must be made by some force—of love, of money, of unquestionable practicality—that was close at hand."

This means that Daisy made a conscious decision not to wait anymore for Gatsby, because she wanted her life to continue immediately. It might have been love or it might have been money that forced her to make the decision. That was when Tom Buchanan came along, and she married him.