This article at Smithsonian Magazine lists some facts about the book, it's reception, the fervor around the creation of the film, and how it broke many, many records. What can be ascertained is that Margaret Mitchell's book pulled a nostalgia from southern history that had been almost unspeakable in light of the Union victory, so there was something appealing about an underdog story. Other articles refer to the fact that housewives bought and read the novel in great numbers and this pushed the sales upward due to a mixed-gender audience that was enjoying it so much.

The expectations around how the film would be made were dramatic: $50,000.00 paid for the rights for the film (a record-breaking amount for that time,) speculation over who would play the lead of the film, over 1,400 actresses auditioned for the role. Some of this hype, along with the sheer expanse of the book and film (over 1,000 pages and the film coming in at 4 hours,) added to the grand scale of the project.

All of this contributed to its success, but the "how" is not really scientific. I believe, based on the data and some speculation, that the backward glance of this lost world was a kind of escapism that suited the grim realities of the Depression Era.