Why does Eugene's mother refer to Elena as "One of those people"?

Elena has developed a massive crush on Eugene, the new boy at school. They start spending more time together, until one day—the 22nd November, 1963—Elena goes round to Eugene's house to study with him. Despite the assassination of President Kennedy, Elena's too excited at the prospect of hanging out with Eugene to share in her mother's sorrow over this tragic event. But when she goes round to see Eugene, she's rudely disillusioned.
When Eugene's mother answers the door, she makes it perfectly clear that Elena isn't welcome. She claims that that there's no need for Eugene to get close to people as he and his family will soon be moving on. But the real reason for Eugene's mother being so unpleasant towards Elena is racism. When Elena identifies herself as living in El Building, the run-down apartment block housing Puerto Rican families, Eugene's mother says "I don’t know how you people do it." By "you people," she means Puerto Ricans. Eugene and his mother are white Southerners and this unpleasant episode illustrates a culture clash between them and their Puerto Rican neighbors, with whom they clearly don't wish to have any connection.