Vivien Leigh was an English actress that had struggled to find her breakthrough in the roles she was playing abroad. I haven't seen a great selection of her work prior to her relocation to the States, but as I understand it, she wasn't necessarily nailed to a type during this time. She transitioned easily from part to part, without having a particularly image attached to her name. That, I feel, was a key to her getting Scarlett O'Hara.
There is a documentary about Gone with the Wind called The Making of a Legend: Gone with the Wind, in which there is an entire section dedicated to screen tests made by the hopeful actresses that auditioned for not only Scarlett, but also for Melanie and Mammie.
Several people more dedicated to a Southerner playing Scarlett campaigned hard for Tallulah Bankhead and Miriam Hopkins, the actress author Margaret Mitchell supposedly preferred. There was a poll that ran in the newspapers, on radio, and fan magazines that said the top contenders were Bette Davis, Katharine Hepburn, Hopkins, Joan Crawford, Margaret Sullavan, and Barbara Stanwyck (I may have the names out of order, though).
All of these women had images already attached to their names, however. As I said above, Leigh did not. She fit the bill in look naturally, but I'm sure there was also an appeal because she wasn't established when negotiating her salary. It also gave freedom for the Hollywood star system to make her in the image they wanted, given her "Vivien who?" status in the US.
She was the right choice, there's no doubt about it, and David Selznick was intrigued by her the first time he met her. Even still, Selznick was still considering four actresses --- Paulette Goddard, Jean Arthur, Joan Bennett, and Leigh --- as late as December 1938. Only Goddard and Leigh were screen-tested in Technicolor on December 20. Of the three outside of Leigh, I can only see Goddard tackling the role with any considerable depth enough to have made Scarlett a standout among the host of great performances that year.
It wasn't to be, though. Leigh got it. And I'm glad she did.