According to your reading in Literary Criticism, Bressler notes that “Marx believed that the history of a people is directly based on the production of goods and the social relationships that develop from this situation” (169). Why is Shevek surprised by the Urrasti economic system given its base and his Marxist views? What facets of the Urrasti superstructure reinforce the hegemony of the Urrasti social system?

In Marxist theory, the economic base consists of the relationship between the producers and the laborers (essentially the employer-employee relationship). The superstructure, on the other hand, consists of the society's culture and governmental power players. Depending on which Marxist perspective we look at, the economic base can influence the superstructure and vice versa.
In Le Guin's novel, despite his Marxist Anarresti views, Shevek is still shocked by the Urrasti economic system. Essentially, in Urras, the power players or ruling elite exist in a world wholly separate from that inhabited by the working classes. The working classes are the dispossessed; their talents are used and exploited by the producers for material gain. Because society is so stratified on Urras, individual citizens often experience alienation within their own culture. On the other hand, in Anarres, citizens co-exist on an egalitarian basis; the practice of mutual reliance and tolerance (at least on the surface) largely powers Anarresti existence on a daily basis.
For his part, Shevek is shocked by the sterilized detachment that characterizes the Urrasti economic system. In Anarres, the practice of interdependence fosters an atmosphere of camaraderie and trust. However, in Urras, Shevek learns that he must keep to himself and learn to distrust those around him. Interestingly, alienated as he is by the culture of "human solidarity" and "mutual aid" in Anarres, Shevek finds himself equally alienated from a culture that trusts no one (as is the case in Urras). Despite his own predilections and inclinations, Shevek finds it difficult to accept the degree of mutual aggression and detachment needed to thrive in a competitive society like Urras.
Meanwhile, the Urrasti superstructure reinforces its hegemony by supporting and maintaining the divide between the producers and the working classes. The power players in the economic sphere are essentially the buyers and the sellers. Shevek notes that the workers who produce the items are systematically erased from public consideration:

The strangest thing about the nightmare street was that none of the millions of things for sale were made there. They were only sold there. Where were the workshops, the factories, where were the farmers, the craftsmen, the miners, the weavers, the chemists, the carvers, the dyers, the designers, the machinists, where were the hands, the people who made? Out of sight, somewhere else. Behind walls. All the people in all the shops were either buyers or sellers. They had no relation to the things but that of possession.

The people he sees on the streets demonstrate disturbing attitudes of impatience and angst. The main concern of everyone seems to be to earn enough to maintain life at subsistence levels. Shevek finds it difficult to accept the surface pleasantries "propertarians" subject their customers to during the course of commercial business; it seems obnoxiously hypocritical to him. He wonders how polite an Urrasti shopkeeper would have been "if he had come in as an Anarresti came in to a goods depository: to take what he wanted, nod to the registrar, and walk out."
Essentially, the superstructure is able to maintain its relevance and hegemonic influence through maintaining its ruling elite's elevated social status as well as reinforcing consuming distrust and latent animosity among its working populace (the dispossessed). With their consuming interest being the need to survive, the working populace has little interest in confronting the superstructure that presides over them.