People tend to like others similar to themselves. In this case, the upper-class people responsible for hiring presumably saw themselves in the upper-class resum

People tend to like others similar to themselves. In this case, the upper-class people responsible for hiring presumably saw themselves in the upper-class resumes, and felt they had a better sense of who they would hire. It's like golf: in business, playing golf made you more relatable to those able to advance your career, and so golf clubs proliferated. Whether you actually liked the sport itself was often a secondary concern.