"Of Modern Poetry" is about one thing: what poetry is supposed to be doing in the modern age. For that reason, Stevens uses the phrase "has to" quite a lot in this poem. The thing is sort of like a manifesto for Stevens' beliefs. In that sense, the entire thing is about the role of literature and writing in society, and what literature should be doing for its readers. For Stevens, the answer's pretty clear. Now that people's faith in religion and science has been shaken, it's now poetry's job to make people feel good about the world and their place in it.
It's crunch time. For Stevens, modern poetry has pretty much run out of options. It's going to have to take desperate measures if it's going to reach people.
According to "Of Modern Poetry," classic poetry was pretty much worthless (in your face, Ovid). Now modern poetry has to pick up the slack and start being relevant to normal people.