I have visited more than 12 bookstores in 3 different states (Kentucky, Missouri, and New York) over the last week. The two African American poets whose books were at every one of the bookstores I visited were Langston Hughes and Nikki Giovanni.
[Related: Blogging about Black Verse--2011]
The presence of their books at those sites was based on the poets' popularity and also the placement power or capabilities of their publishers.
Hughes's books are published by Vintage and Knopf, which are now divisions of Random House. Giovanni's books are published by William Morrow, which is now a division of HarperCollins.
The big publishing difference early in Hughes's publishing career was Knopf, and for Giovanni, it was William Morrow. During the 1920s/1930s, Knopf gave Hughes an extraordinary boost, ensuring that his books were kept in print even during the Depression. For Giovanni, the big boost came from William Morrow when they published two of her previously published works Black Feeling Black Talk and Black Judgemen as one book in 1970.during the black arts era (60s/70s).
Over the decades, the popularity of the poets has persists. Thanks to some of the poets' signature poems and the tremendous support they receive from their publishers. It's why books containing their poetry show up in so many bookstores.