Why, do you think, Herbert Spencer's idea of "Survival of the Fittest" appealed to many in the middle class at this point?

Can a "good business" be an unethical business?
December 14, 2017
Economics of Environmental Quality
December 14, 2017

Why, do you think, Herbert Spencer’s idea of “Survival of the Fittest” appealed to many in the middle class at this point? What part did the rapid industrialization and interpretation of the 14th amendment play in this philosophy, if any?