When it all comes down to it, Wallace Stevens doesn't want to write about beautiful things like flowers and love. He wants to write about how human consciousness goes about finding beauty in the world, which is a different thing. Different people will find different things beautiful. But there are similar processes that happen in our minds when we find pleasure in a poem or in an experience. In "Of Modern Poetry," Stevens wants to talk about this common denominator. He wants to talk about the process of finding beauty, not beauty itself. And that means he wants to talk mostly about how the human mind works and how we perceive the things around us.
For Stevens, some things are more worthy of being called beautiful than others (we're looking at you, rainbows). It's not all subjective.
In "Of Modern Poetry," Wallace Stevens shows us that the nature of human consciousness has totally changed in the modern world. We need new poetry that can speak to our new collective nature. So get right on that, why don't you?