I know now, thanks to this very interesting Political Affairs interview with author and CUNY professor Gerald Meyer. The whole is worth reading but here are some snippets.
When we think of portraiture, it seems like an elite genre of art. Portraits are painted of kings, of very rich people, people who want themselves memorialized. There is often an element of glorification in it. But Neel takes that art form and applies it to poor and working-class people, minority people, dignifying them, individualizing them, and immortalizing them. It is very moving. You get a sense of them as social actors, as part of what is going on in society, and that they are, in fact, part of something that commands our attention. ...I find this last point particularly refreshing, since social (or socialist) realism is so uniformly trashed by prevailing trends of bourgeois cultural criticism. It buoys me too, since it fairly well describes what I'm mostly trying to do in my own writing. And of course it all makes me desperate to see Neel's paintings next chance I get.
In social realism, there is something beyond description which points in a certain direction, in a direction that makes us say that what we are seeing is unacceptable and therefore requires change. Or perhaps in what we are seeing or reading about, or watching in the theater, we get the sense that the subjects themselves are going to do something about the situation they are in, that it is not tolerable. Therefore, the art becomes critical of reality, so it is not just a description of reality but a description that embodies a criticism of it. I think you have that in Neel. There is a sense that these people don't deserve to be poor, and that they are capable of doing something about it. They are not just victims.
Meyer has a longer piece,"Alice Neel: The Painter and Her Politics," in the Columbia Journal of American Studies, which I'm going to read when I can grab the time.