It seems that the canonization process is completely artificial. Critics have come up with their lists as to what should be considered valuable writing. And their evidence is to be seen within the pages of the well written plays, poems and whatever else. Read the works for proof of their value.
But why should Milton be considered foremost, which puts him in the center of how criticism should operate? Look at Milton's work and compare that to the plays of Shakespeare to see how differently these poets wrote. Shakespeare made characters, while Milton wrote criticism.
Milton gave us a tradition of being self-aware of one's position within what we consider to be canonical writing. There is nothing from Shakespeare analogous to what Milton wrote within his political tracts about his desire to give posterity a type of canon. He seems to have an edge here.