I'm having real trouble with this question.
I can answer your question in one word: security.
It's what any member of the "old rich" truly wants. I'm afraid I can't give Daisy much credit for her character; however, I must admit that there was at least a little bit of love for Tom somewhere in the past and, in my opinion, the view through the window of the two of them holding hands after the Myrtle accident shows me that, just maybe, that little bit o' love is being rekindled.
Another thing that I must mention, but I'll admit is simply me reading between the lines, is that Daisy just might have some fear associated with her husband. Remember at the beginning of the novel when Tom had previously hurt her finger and Daisy calls Tom "hulking." Uh huh. A bit of physical abuse? Maybe. Who is to say there won't be more, ... especially if she continues to threaten divorce.
As for Gatsby, remember, it might not be love quite as much as an obsession. Does Daisy notice this? Hmmm, probably not, but my guess is she feels there's something just a little bit creepy about the whole thing. Remember, Daisy is the one who yells at Gatsby: "You want too much!" Uh huh. Very telling.
It's not just that Gatsby is low class. It's also that he's actually been involved in criminal activities. That is beyond low-class, right? So I agree that Daisy is still too snobby to be with Gatsby, but I think it's important to note that Gatsby was willing to do illegal things to get money and that could be a problem for Daisy.
Also, I think that Daisy really doesn't love anyone but herself. So all she's going to care about is what is going to be best for her. Tom is just as rich as Gatsby and is better connected socially so Daisy will probably have more of the sort of life she wants with him than with Gatsby.
Daisy is an aristocrat who has lived her whole life among the "old money" people.
While Gatsby has become wealthy, he is what we call "new money" people. He has money, but he lacks the elegance and refinement that those who have been raised in money have. He has longed to be part of that world his whole life, and has often admired Daisy because she has the grace and elegance that "old money" breeds. He even remarked that she had "money in her voice."
However, even though he loves Daisy's world, he will never be a part of it, and he loses her for the same reason that he adores her: her high money arrogance.
Daisy may not love Tom as much as Gatsby, but she cannot bear the thought of living in the low class world of "new money". So, she chooses the world she knows (Tom) over the world of new money (Gatsby).
Ermm, I'm not sure why everyone is talking about class. If Gatsby being new money was a problem why did she say that she was going to leave Tom for Gatsby? It was only when Tom unveiled Gatsby as a bootlegger that Daisy rejected Gatsby. She didn't want to be with a criminal- despite the fact he only exploited the ridiculous law for her.