Thinking back on the photo, I was reminded of a portrait of Catherine the Great I saw at a recent museum exhibit: in terms of subject matter, the portrait is the most standard, official-picture type of thing. Subject in center, light coming from 45deg and above, dark background, and some objects here and there. However, the objects were carefully selected to make a symbolic statement about her reign. Most official portraits use the same technique: make it plain, but write your meaning with picture details.
What does it have to do with photo? Well, because we consume photos much more often, and much more faster than we do painting, we tend not to "read" them. Also, the symbolic-object technique is not always a part of photomaking, given that photographers often picture what is available, rather than always carefully choosing the details.
We tend to linger less on "reading" the photos. With a disarmingly simple picture like Eggleston's, the slower viewing method may be more appropriate. For instance, some people have mentioned the composition of Eggleston's photos resolving into the american South flag.
A caveat of this traditional fine arts appreciation is reading too much into a photo. We all know the enormities that a piece of concrete can generate in a gallery. Trapping the viewer by offering meaninglessness where meaning is expected wasn't as common before the 20th century as it is.