So nothing holds the molecules together so they could freely escape into the environment if they have the kinetic energy. However, in a non ideal fluid there are intermolecular forces and these forces hold the molecules together. They inhibit the molecules from escaping into the environment. So at any given time, less molecules will be able to escape in a non ideal fluid than in an ideal fluid. And because there are less molecules present in the states above the pressure will be less. Therefore, the stronger the intermolecular forces, the less likely that they will evaporate.