Failure of the paint to resist absorption of dirt and stains.
Use of lower quality paint that is porous in nature.
Application of paint to unprimed substrate.
Higher quality latex paints contain more binder, which helps prevent stains from penetrating the painted surface, allowing for easy removal. Priming new surfaces provides maximum film thickness of a premium top coat, providing very good stain removability.
(image courtesy of The Rohm and Haas Paint Quality Institute)