Why does Daisy choose Tom over Gatsby?
To understand why Daisy chooses Tom over Gatsby, it's important to consider that Daisy probably values her own happiness and comfort more than anything else. While she undoubtedly loves Gatsby, running off with him would be very uncomfortable indeed. Her reputation would be tarnished, and though Gatsby is rich, she would still have to change her lifestyle and adapt to a new mode of living. It goes without saying that all of this would be very uncomfortable. As a somewhat shallow person focused on material comfort, Daisy is ultimately unwilling to leave with Gatsby because she is unwilling to upset her preferred status quo.
It's also worth considering that Gatsby asks too much of Daisy. Instead of merely asking her to leave with him, Gatsby urges Daisy to declare that she never loved Tom. Again, while Daisy loves Gatsby, she's also had a close relationship with Tom for many years. As such, the world is not as black and white as Gatsby would like it to be, and Daisy's love life is far more complicated than he imagines. In that case, another reason Daisy might stay with Tom is because she cannot bring herself to utterly condemn him.