Compare and contrast why Daisy doesn't want to divorce Tom and marry Gatsby.

There are a number of reasons why Daisy won't divorce Tom and marry Gatsby. For one thing, she actually takes her marriage vows seriously and so never really contemplates divorce, no matter how often Tom cheats on her. Also, Daisy has grown rather accustomed to the old money lifestyle that her marriage provides. Despite her regular appearances at Gatsby's lavish parties, she still has something of a snobbish disdain for the nouveau riche West Eggers:

She was appalled by West Egg…by the too obvious fate that herded its inhabitants along a short cut from nothing to nothing.

For all the many ups and downs of her marriage, Daisy feels much safer in her comfort zone. Marrying Gatsby would be too much of a leap in the dark, and Daisy just doesn't like taking chances.
Daisy's relationship with Gatsby is also more than just a tad one-sided. Gatsby's never really got over Daisy, or at least the idea of her. Daisy, however, has moved on from their earlier relationship, so much so that when Jordan mentions Gatsby's name at a dinner party, she doesn't even know who she's talking about. Their relationship from five years previously meant everything to Gatsby, but clearly nothing to Daisy.
Even when they get back together, Daisy seems more emotionally engaged by Gatsby's fine collection of shirts than by the man himself. But Gatsby's so blinded by his love for Daisy that he's incapable of seeing that there's no chance that they'll ever be together on a permanent basis. No wonder poor old Jay finds himself obsessively staring at the end of Daisy's dock, reaching out forlornly to the green light.