I got two similar answers: Daisy doesn't leave Tom because the two of them are well suited for one another - they are alike. One of the clearest examples of this similarity is in Chapter 7, after Daisy and Tom are back at their house after the disastrous day in the city. Daisy accidentally hit and killed Myrtle with Gatsby's car as Daisy drove home from New York following the confrontation between Jay and Tom. The reader sees Daisy and Tom, with their heads bent over a table of untouched food, talking. The scene that Fitzgerald paints here with his description shows two people who are bookends, holding up their careless lifestyle between them. Daisy is a spoiled, petulant woman-child. In Chapter 8, when the romance between Jay and Daisy is recalled, the reader sees that Daisy is upset with Jay because he has to be gone due to the war. She is so self-centered, she doesn't understand why he doesn't just come home and spend his time with her. Throughout the book, the reader sees the narrow-minded and self-centered Tom. The biggest difference between Tom and Daisy is the way each displays this quality. Tom uses aggression (he's described as a bully and we see several examples of this). Daisy hides behind her dreamlike quality; her vagueness
Second Answer: In The Great Gatsby, Tom is presented in a negative way. First, though, remember that the narrator is the one characterizing Tom, and he doesn't like Tom. This is clear from the first time he describes Tom in chapter one, when he reveals what he expected Tom to be like before he met him during the course of the novel's story. Nick knew Tom in college, and though the novel's story occurs years later, Nick carries over his dislike for Tom from his college days. Daisy does at least indirectly reveal some of Tom's negative personality traits, but even her words and actions are filtered through the the narrator, since he is the one writing them down. And Nick is not necessarily a reliable narrator.
This is not to say that Tom isn't somewhat as Nick portrays him. He probably is. But in terms of Daisy staying with him this is not a central issue. She doesn't decide to stay with him because he is or isn't a good guy.
Daisy stays with Tom because he is the status quo. Economically and socially, Tom represents old money, inherited money. Tom is on the top of the food chain, so to speak. He is of the elite class. Daisy must stay with him to maintain the status quo. Gatsby is wealthy, too, of course, but he is an upstart. His money is new. He does not have a respectable family name, etc. His family is not well established.
Specifically, though, Daisy might have been talked into leaving Tom for Gatsby, except that, as she says, Gatsby demands too much. Gatsby insists that Daisy make an announcement that she never loved Tom and that even when she married Tom, she was really pining for Gatsby. And Daisy won't lie--at least not about this. She did love Tom once, and she won't deny that. In short, Gatsby sees the relationship they had five years earlier as special and poignant, as something earth shattering, but Daisy doesn't. Gatsby was just one suitor among many, apparently.
Daisy stays with Tom, then, to maintain the status quo and because Gatsby asks too much of her