Mollie is one of the horses on the farm who has a very unique personality to that of the other animals. Unlike Boxer, who has a tireless work ethic and a devotion to the toil of labor and production, Mollie craves attention and her personal luxuries. To fully understand the answer to this question, one needs to explore Mollie's most critical trait: her vanity.
Mollie craves attention from humans, loves the taste of sugar cubes as a treat, and takes pride in her appearance by wearing ribbons in her mane. With these traits in mind, it is easier to understand her reluctance in accepting a new life without her important luxuries. She is in disbelief that her precious sugar will not be available on Animal Farm, as seen when she asks, incredulously, "Will there still be sugar after the rebellion?"
She further asks Snowball if she would be allowed to wear the ribbons in her mane. Snowball, in an attempt to combat her confusion, says, "Comrade...those ribbons that you are so devoted to are the badge of slavery. Can you not understand that liberty is worth more than ribbons?"
You see, Mollie cannot. She cannot understand that the new principles on Animal Farm deviate from and contradict her own ideals. Therefore, Mollie is unable to adjust to this new life. She did not share the same sentiment that humans were oppressive. To her, they were an audience to which to show off her beautiful ribbons. She did not share the belief that all animals should be equal; she prefers the splendors that come with pulling Mr. Jones's carriage.
For further research on Mollie's materialism, see the link below to another eNotes question that explores Mollie as a symbol of Western liberties and capitalism. This could help you gain a deeper insight into the character's behavior before and after the revolution.
Representation of the horses in Animal Farm