Why were the concepts of "racial fitness" and Social Darwinism so popular in their era?
These concepts were very popular during the late 1800s and early 1900s because this was a time during which there was a major gap between rich and poor in the United States. In addition, it was a time of increased immigration of groups (Eastern and Southern Europeans) who were not previously very common in the US.
As the gap between rich and poor grew due to industrialization, people needed a way to justify the change. Social Darwinism became popular as a way to justify this seeming inequity. With Social Darwinism came the idea that there was a need for "racial fitness." Theodore Roosevelt and others like him felt that the "Anglo-Saxons," or Americans of Northern and Western European descent, needed to maintain and improve their fitness so that they could continue to dominate the "New Immigrants."