Commercial works must appeal to large, massive audiences, and so tend to have themes, characters, situations and conflicts that are more general, universal and relatable. "The Man Who Was Almost a Man" tells a very specific story, about a very specific character and situation, and does it in a way that is a bit more difficult to understand or relate to. For example, Wright uses dialect, which is the actual way that people talked. The dialect of the sharecroppers is very difficult to understand; it takes concentrated reading. Commercial products don't take that kind of effort--they are easier to grasp. Also, writing dialect takes a lot of talent and literary skill.
In addition to the dialect, the story of a stubborn, obnoxious and immature kid who kills a mule then skips town doesn't have too many themes or likable situations. When I have my students read the story, many of them don't like it because the mule died (that's a sad ending; commercial tales have happy endings), and they don't like Dave (typically, commercial tales have protagonists who are heroes, or likable). So, to take an unlikable protagonist doing unlikable things, you aren't going to get mass commercial appeal. However, the dialect, the style, the layered themes, the depth and symbolism that exist in the story all make it very literary and worth studying. I hope that helped; good luck!