They believed in the survival of the fittest theory, which in social theory meant that they believed that people who were poor were not fittest and should be left behind to make way for those more successful.
Social Darwinists viewed poverty as a natural phenomenon that was meant to happen. They thought that assisting the poor would disrupt natural selection in society, and believed in the motto "survival of the fittest."
Social Darwinism argues for the survival of the fittest and therefore if someone would be come not fitting in the society, then for this point of view they should be left behind to let the more successful group of people survive and grow. Hope this answers the question.