In The Great Gatsby, why might Daisy choose to remain with Tom instead of leaving him for Gatsby ?
In The Great Gatsby, Daisy stays with Tom because, in her words, Gatsby asks too much.
Gatsby demands that Daisy tell Tom that she never loved him, that all the time she was married to Tom she really loved Gatsby. She loved Gatsby five years before, she admits, and she loves him now (the present in the text), but Gatsby insists that she loved him at all times in between. When Tom reminds her of some of the good times they had together early in their marriage, she refuses to say that she never loved Tom. That's when she tells Gatsby that he asks too much.
Gatsby's illusion is that he and Daisy have a special, poignant relationship. His illusion could stay intact if Daisy married Tom for a reason besides love: for financial security, for instance. But if she married Tom for love, or just loved him at all and at any point, his illusion would be destroyed.
He insists on the truth of his illusion, but his illusion is not true, and Daisy refuses to say that it is.
Daisy, like most women, needs security. It's noted that Daisy wouldn't divorce Tom because she's Catholic. In religion there is security whether real or imagined because there is explanation for why things happen the way they do.
Besides being financial security for Daisy, Tom provides a stability of familiarity and I think the evidence of his learning about Daisy and Gatsby demonstrates that he also provides true love. Sometimes new love (like Gatsby) is alluring, but there is a reason marriage is a building block of society. It is a growing process of stages. My guess is that post-book, Daisy and Tom find a rekindling of their first love for each other and rely heavily on each other for friendship as well as a physical relationship. Trials in a marriage make it stronger if we get through them. I can't imagine a greater trial than this.
I think that there are probably a number of reasons why Daisy would remain with Tom, but I think the most important one has to do with how Gatsby has made his money.
It seems pretty clear that Daisy loves Gatsby much more than she loves Tom. However, her feelings for him seem to change as Tom talks about the illegal ways in which Gatsby has made his money.
This shows us how Gatsby's quest for Daisy has gone wrong. He is pursuing someone or something that really does not care about him. She cares much more for money and image than she does about him personally.