Because Maleficent is not actually evil.
Her bad deeds were a direct result of her reaction to Stefan's betrayal of her. She followed Aurora's life to revel in her "ingenious" revenge. Maleficent kept Aurora safe initially (feeding her and keeping her from falling off a cliff) so that she could continue to enjoy her plan.
Of course, over time her normal, loving personality became dominant, and she wished she had never cursed Aurora. She tried to reverse it. Eventually, her guilt and love drove her to make Aurora the center of her life. Another factor would be that Aurora was given the magical gifts of being happy and pretty (well liked by others).