Is there some reason that you believe you ought to be able to find a protagonist in this book? In general, you would not say that a non-fiction work (especially one that is mainly statistical as opposed to, say, a biography) has a protagonist.
In a work of fiction, the protagonist is the hero -- the person that the reader or audience is supposed to identify with. Is there a person in this work that you are meant to sympathize with? That's how I would look for a protagonist.
I suppose you could say that blacks in general are the protagonist. Or that the individuals whose stories are told are the protagonists. But I fail to see how you could identify an individual who is the protagonist for the whole book.
As the previous post indicates, it is difficult to identify a protagonist in a non- fiction work. Statistical breakdown like Wells' work does not center around one particular individual, but on a condition or institutional challenge present. Yet, this does not stop us from playing around with the idea. Perhaps, the protagonists in this setting could be those who are being threatened with lynching. Another potential protagonist could be the Progressivist thinkers like Wells who seek to eliminate the problem of lynching in an industrialized America. Even more relevant could be the society that allows lynching, and Wells' work is calling out to them to change what is and transform it into what should be.