Why are Countee Cullen and Lois Mailou Jones considered to have been major contributors to the Harlem Renaissance?

The late poet Countee Cullen is, indeed, well-known. However, Lois Mailou Jones's name does not frequently come up when one talks about the Harlem Renaissance. Really, the only female name that does frequently come up is that of Zora Neale Hurston.
Despite the major contributions of women to the Harlem Renaissance, many of their names are less-known (e.g., Jessie Fauset, Nella Larson) or unknown. We mainly hear the names of men (e.g., Langston Hughes, W.E.B. DuBois, Claude McKay, Alain Locke, Arna Bontemps, Wallace Thurman, and that of Cullen). This reveals that this movement was no less prone to sexism than any other.
Nevertheless, I would agree that Jones was a major contributor. Before the Harlem Renaissance, black painters followed European style convetions. Henry Ossawa Tanner, for example, was a major painter in the late-nineteenth century. His work followed the conventions of Impressionist painting. Some of Jones's work also follows these conventions, such as her portrait, Babella, Paris and her still-life Cauliflower and Pumpkin.
However, by the 1930s, her art began to reflect an interest in African art, particularly masks. Her exploration of African art motifs not only reflect a generally Modernist interest in African aesthetics (e.g., the works of Pablo Picasso and Francis Picabia), but also the goal of Harlem Renaissance artists to pursue a uniquely black aesthetic, a pursuit that would not be seen again in America until the Black Arts Movement of the 1960s. 
Countee Cullen is considered to be one of the finest poets of the Harlem Renaissance, treating themes, such as racism, with great sensitivity (e.g., "Incident"). During his time, however, he was criticized for his adherence to classical verse and for his Romantic influences. Again, one of the goals of the Harlem Renaissance was to legitimize a uniquely black aesthetic. Some saw Cullen's willingness to eschew black idiom as a denial of his own heritage. On the other hand, his choice of conventional forms probably also earned his work more attention among mainstream audiences.
 
https://www.britannica.com/biography/Countee-Cullen

https://www.britannica.com/biography/Lois-Mailou-Jones