"To serve" means "to provide service for." Providing news about Boston is a useful service to people who live in Boston, but not to people who live in Milwaukee. The Boston Globe serves Boston; it carries lots of Boston news. The Milwaukee Journal services Milwaukee; it carries lots of Milwaukee news.
Some people live in towns that are not served by any newspaper. They have to read a newspaper from some nearby town and hope that it carries some news from their town.
Your sentence means that people who buy newspapers are becoming more and more diverse, and therefore newspapers will be interested in hiring writers that reflect that diversity. Therefore there should be job opportunities for minorities, writing for these newspapers.