Stevens wrote "Of Modern Poetry" at a mature point in his career. By this time, he wasn't all that keen on using strict forms and meters. Instead, his poetry tends to sound the way someone would just say something to you in plain language.
That said, Stevens uses some really solid enjambment to lead you from one idea to another in this poem. In fact, it's tough to find a line in this moment that doesn't use enjambment. Why does Stevens do this? Well the very first line provides us with a bit of a clue. Stevens claims that his poem is about "the act of the mind finding/ What will suffice" (1-2). The first line falls off on the word "finding," which sets a searching tone for the entire poem.
The rest of the poem, then, mimics this movement of a mind that is searching for something, always about to get there, then falling into the next line. Stevens is interested in using specific images and memorable phrases to convey his ideas about modern poetry, and is more than happy to do this without the help of traditional forms. Let's not forget that the subject of this poem is how modern poetry should try to move beyond classical forms. So it makes sense that Stevens would go with the openness of free verse and write the lines whatever way he wanted.