Caillebotte's work was probably rejected because it is not a great narrative "show" or a majestic landscape. Instead, the painting concentrates on just a few details and gets them perfectly right. Caillebotte demonstrates no interest in the walls; they are dimly colored, reflect no light; they simply confirm the indoor setting. Instead, the men are lit, and we see a hint of their conversation (in the tilted head); we notice the ironwork of the window and the curls of wood. For all its gorgeous light, this is a rather claustrophobic, hard picture, with the promise of hours of work remaining for the three workers.
Suzanne Langer wrote that "An artist .... objectifies the subjective realm. What he expresses, therefore, is not his own actual feelings, but what he knows about human feeling. Once he is in possession of a rich symbolism, that knowledge may actually exceed his entire personal experience. A work of art expresses a conception of life, emotion, inward reality. But it is neither a confession nor a frozen tantrum; it is a developed metaphor ... " (from "Expressiveness," in The Problems of Art). I love the term "frozen tantrum," (earlier, Langer had written that a baby's cry is not art); this is a painting thought through over time, finished over days, weeks, perhaps longer than the men will take to do their work. We wonder how the artist came across these men, this image, which is certainly, as Langer says, not the artist's "confession" at all. This painting is portraying a moment that viewers might not otherwise, ever, have "seen.". The "developed metaphor" here gives us a way of seeing work that is not ours, and ... the picture does become the kind of painting that the Salon all those years ago demanded. It is a narrative; it is majestic in its own fierce, concentrated way.
Caillebotte has given us a permanent reflection, here; one set of feelings (his) have been exchanged for another (that of the workers). It isn't his life, but for as long as we look at the painting, it is ours.