The King and I by Rogers and Hammerstein, itself based on Anna and the King of Siam by Margaret Landon, itself a fictionalized version of The English Governess at the Siamese Court by the real-life Anna Leonowens, based on her experiences at Siam. That's four levels of adaptation, folks.
In 1951, Rogers and Hammerstein adapted a 1944 novel by Margaret Landon, based on the memoirs of Anna Leonowens, an Indian-English-American-Canadian educator and her time in the royal court of Siam (Thailand) in the 1860s into a successful stage musical starring Yul Brynner in the role of The King, which become his trademark role.
A 1956 film also starring Brynner and Deborah Kerr, would win several academy awards.
Following the success of Anastasia and The Prince of Egypt, which were also based on true stories and adapted into movies starring Yul Brynner in the 1950s, and the announcement of a serious drama starring Jodie Foster and Chow Yun-Fat based on the story, probably propelled this film to be made.