Very fair criticism, I think when I was putting this together I went the mainstream route because I wanted to get broad appeal from people who had never even given the genre a chance, but thinking more about it your suggestions make a lot of sense (i.e. moving through different periods).
Hip hop, by definition, at least to me, is more of an underground culture evolving from jazz, funk and poetry. I also think most people have heard these songs because they are pretty widespread on the radio, like Lil Wayne, Jay-Z, Outkast, Drake, Beastie Boys... etc.
A better introduction would be to move through different hip hop periods to show how the sound has evolved as the genre has matured, including artists that are actually in it for the art and not solely for the money.