Simply put, I am not sure the concept and idea of the poem works as well if it is written in paragraph form. I don't think that Hughes is trying to construct an essay, or some type of expository piece about the nature of people of color in Western History. It is for this reason why the poem has to be a poem. It has to be something whose tonal lyricism can be evident in form. Hughes wants to create a voice or condition that spans time. In doing this, it has to transcend the idea of formalism and structure, a voice that is transcendent. This is able to be evoked in the poetic manner, one in which the reader is more predisposed to allow a voice that can speak of the different time epochs in human history as having seen the past into the present and the future. In an expository and paragraph form, the reader is more likely to ask questions, preventing this voice from emerging in its full reflective and transformative setting. In being able to use the lyric poem form, Hughes enables this voice to become more real and definable in a sphere where the reader is left to ruminate and reflect upon what it means to be a person of color in world history and in the current context in which Hughes writes.