When poets refer to other great works, people, and events, it’s usually not accidental. Put on your super-sleuth hat and figure out why.
It's not totally clear what Stevens means when he says that the theatre of poetry changed. But he's probably referring to some major modern event that transformed the nature of poetry. In this sense, he most probably means the two world wars, considering that he was writing this poem just as the Americans were entering WWII. (5)