What makes Frederick Douglass' "What to the Slave is the Fourth of July" special to American Literature?

Part of the reason Douglass' comment on the 4th of July is so powerful to American Literature because few other works specifically show the gulf between what is and what should be.  Douglass artfully praises the framers of the nation for creating the language that displays the fundamental right to be free and for developing the vocabulary that displays what it means to be a dignified human being.  Then, he abruptly punctuates it with a counter point:

I say it with a sad sense of the disparity between us. I am not included within the pale of glorious anniversary! Your high independence only reveals the immeasurable distance between us. The blessings in which you, this day, rejoice, are not enjoyed in common.  The rich inheritance of justice, liberty, prosperity and independence, bequeathed by your fathers, is shared by you, not by me. The sunlight that brought light and healing to you, has brought stripes and death to me. This Fourth July is yours, not mine. You may rejoice, I must mourn. To drag a man in fetters into the grand illuminated temple of liberty, and call upon him to join you in joyous anthems, were inhuman mockery and sacrilegious irony. Do you mean, citizens, to mock me, by asking me to speak to-day? If so, there is a parallel to your conduct. And let me warn you that it is dangerous to copy the example of a nation whose crimes, towering up to heaven, were thrown down by the breath of the Almighty, burying that nation in irrevocable ruin! I can to-day take up the plaintive lament of a peeled and woe-smitten people!

Douglass' fundamental claim is revealed in this passage when he argues that a nation predicated upon the drive for individual freedom and human dignity is also the nation that enslaves people for profit, denying them their own sense of autonomy and freedom while celebrating its own.  Douglass' work, and in particular this speech, is essential to the American Literature lexicon because it reveals the notion of "multiple Americas" throughout American History.  In a nation whose history and literature denies the institutionalized stratification of others, slavery existed.  In a literary tradition that emphasized the promises and possibilities of freedom, discrimination and silencing voices was present.  In a house where all are stated to have "inalienable rights," political, economic, and social marginalization was, and to some extent, currently is the norm.  The belief in what America can be is strongly negated when reading Douglass' words.  While no one will deny Douglass' worth to the American literary tradition in his writing's high level of quality and technical skill, it is his statement and demand to include multiple and varied voices into the canon of American Literature which is the reason why this speech on the 4th of July is so essential to it.