Why did American writers gradually move away from Romanticism towards Realism in literature?

This question has fairly broad philosophical implications, but to answer in brief, it is useful to remember that any kind of art is not created in a vacuum; art reflects the time and place in which it is created. Artistic movements (visual, literary, musical) inevitably arise as a rejection of what preceded it.  Just as the Romantics rejected the cool rationalism of the Enlightenment to embrace a more mystical, emotional, spiritual, and imaginative perspective that idealized Nature, Realists moved away from these concerns toward a renewed interest in rationalism, along with verisimilitude in depicting the everyday lives of ordinary people. Industrialization, a growing and diversifying population, and the expansion of urban living led writers to create work that explored the enormous social changes as the country began moving away from a heavily agrarian system to a more open and democratic society.