The question of why Daisy would stay with Tom instead of leaving him for Gatsby is a very interesting one. There is plenty of evidence and subject matter to use in a discussion on this topic:
- Daisy yearns to fulfill a romantic vision with Gatsby but fears the unknown future with him. Daisy is dazzled by
- Gatsby's wealth, yet refuses to erase the significance of her past and refuses to say that she never loved Tom.
The two most likely reasons that Daisy will not/would not leave Tom for Gatsby seem to be that 1) she cannot bear the idea of negating her entire past with Tom and 2) Gatsby turns out to be unreasonable and domineering in ways similar to Tom.
When Gatsby demands that Daisy say she never loved Tom, he is being as cruel as Tom ever was. His concern for Daisy is proved here to be shallow.
However, Daisy is not able to make a final decision about her future before she kills Myrtle with a car. In doing this, Daisy's circumstances change. Her emotional state is no longer strong enough to face the idea of a big change like leaving her husband. She recedes immediately into a defensive place, by Tom's side.