It tells us a lot about Southern society at the time that only a white person is able to tell the stories of African Americans. Aibileen and Minnie have one heck of a story to tell, but until Skeeter arrives on the scene, no one's interested in hearing it. As well as their lowly status as domestic servants, their race precludes them from being taken seriously by white society. The idea that African Americans—servants or otherwise—have anything important to say simply doesn't occur to many white people in this part of the world.
That's why it's deemed necessary for someone to whom white society can relate (i.e., Skeeter) to tell the women's story. This makes Skeeter's task all the more difficult. On the one hand, she has to shape the women's narrative so as to make it palatable to a largely white audience. On the other, she must do justice to the women's stories and convey the full range of their experiences with the utmost fidelity to the truth.