Why did Barracoon remain unpublished for nearly a century?

In her Introduction and Afterword to Barracoon, editor Deborah G. Plant mentions several reasons for the delay in publication. Hurston’s original research had been commissioned by Carter Woodson, and she was obligated to publish the material according to his instructions. An article was published in The Journal of Negro History in 1927. Disputes with Woodson over ownership of the material prevented further publication.
Hurston expanded the material into a book-length manuscript and worked toward publishing it, in part through correspondence with Viking Press. Hurston recorded much of the narrative as closely as she could to Cudjo’s voice, in her understanding, which was dialect. Viking, according to a letter from Hurston cited in the Introduction (xxii-xxii), wanted it written “in language rather than dialect,” and Hurston would not revise the manuscript accordingly. In addition, Plant suggests, publishers might have been “unwilling to take risks on ‘Negro material’ during the Great Depression.”
Hurston, Zora Neale. 1927. "Cudjo's Own Story of the Last African Slaver." The Journal of Negro History, 12(4)(October): 648-663.
https://www.harpercollins.com/9780062748201/barracoon/

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Zora Neale Hurston’s book Barracoon remained unpublished for nearly a century for two reasons. First, publishers thought it would be too hard for the average person to read. Second, it raised uncomfortable questions about the slave trade.
When Ms. Hurston interviewed Oluale Kossula, who also used the name Cudjo Lewis, she transcribed their conversation in a way that depicted in writing the vernacular Kossula spoke. This was one rule of the ethnographic interview project conducted by the Works Projects Administration, for which Hurston was working at the time. The result, however, was difficult to read even for professionals. It was feared the public wouldn’t buy the book, because they wouldn’t understand it.
Kossula told the story of his capture in Africa, which wouldn’t have been possible without the cooperation of the chiefs and warriors in Dahomey, which is part of the West African country of Benin today. The notion that black people helped cause the enslavement of other black people wasn’t a popular view in the US after the beginning of the Civil Rights movement. It is still contentious today.