Fitzgerald provides several pieces of evidence to support Nick's assumption that Daisy married Tom for and will never leave him because of his wealth.
1. First, in Chapter 1 when Nick visits his cousin Daisy after not seeing her for several years, he witnesses the awkward situation between Tom and Daisy over Tom's mistress. While Tom is away from the table, Nick tries to lighten the mood by asking Daisy about her little girl. Daisy becomes more melancholy, but then Nick watches her snap out of it, and he observes:
"she looked at me with an absolute smirk on her lovely face as if she had asserted her membership in a rather distinguished secret society to which she and Tom belonged" (22).
Nick's point is that because Daisy has "seen everything and been everywhere" with Tom's money and has gotten used to covering up problems by using their wealth, she is unwilling to leave even an abuse- or adultery-filled marriage.
2. In Chapter 4 when Jordan meets with Nick to ask him on Gatsby's behalf to invite Daisy to his house, she explains to Nick the history between Daisy and Gatsby. When she finishes telling Nick about the couple, she mentions that Daisy's affection for Gatsby was stifled by her family and that not too long after Gatsby goes to war, Daisy was happy again because
"in June she married Tom Buchanan of Chicago with more pomp and circumstance than Louisville ever knew before" (80).
All of Fitzgerald's comments about Tom and Daisy's courtship and wedding relate to wealth. It is truly the only aspect of Tom's character that can be considered remotely positive or persuasive enough to make Daisy forget her soldier and move on.
3. Most significantly, Nick's final judgment of the Buchanans, that
"they were careless people . . . [who] smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together" (187-88).
demonstrates that he has no other explanation for why Daisy would stay with Tom other than their money and his ability to be able hide not only his own scandals but now also hers by paying people off, picking up and moving, or simply denying that they did anything wrong (all made possible because of their abundant, seemingly infinite access to money).