Why was early Christianity so obsessed with the rich?

Here’s one explanation, from Through the Eye of a Needle (p. 56) by Peter Brown:

Even for the most courageous, many topics were taboo. Reading the stirring sermons of a bishop such as Ambrose of Milan, for instance, one has the impression that denunciations of the rich in general were a substitute for criticisms of the policies of the court in particular. In a court city such as Milan this was a real issue. Ambrose once had to intercede for a man who was being led off to be executed for treason simply because he had said that the present emperor (the young Gratian) was unworthy of his father, Valentinian I. As a result, social criticism tended to be general and studiously non-specific. One suspects that the evil rich—invariably presented in tracts and sermons as generic monsters of avarice and luxury—acted as the whipping boys of public opinion in a world where more precise grievances (such as resentment over high taxation and ill-advised political decisions) could not be expressed.