The image of the "sacred white" woman reappeared in motion pictures in the wake of Brown v. Board of Education, according to Allison Graham. She symbolized a "threatened and embattled way of life," and white Americans,
Invoking a fantasy to defend a fantasy, segregationists rallied around their favorite icon...'a well-bred, cultured Southern white woman or her blue-eyed, golden-haired little girl.'
In short, segregationists fell back on this image, which appealed to whites around the country, in order to create an image of something worth defending against integration, which they associated with miscegenation and racial destruction. So virginal white women took their place alongside ignorant white rednecks as symbols of the South. Both, as has been alluded to in other answers, were evocative tropes for Hollywood feature films precisely because both could arouse white anxieties.