Since the foundation of America, the idea of an unmarked 'frontier' always captivated the minds and hearts of people. People had this idea of freedom where they could always resettle away from the cities and migrate to the frontier to start a new life.
Whenever the frontier opened up, thousands of people found new fertile land and started a new life.
Unlike in European cities where poverty and crime would create deadly protests, the US would just keep on expanding to the west and provide more for the people.
With the end of the Frontier, there was no new land nothing for rural person to look forward to.
However, the problems of poverty and social cohesion could easily be solved by providing good education and better jobs to people.
The US had to move on from an agriculture society to an industrial one and eventually develop an economic system that provides for all and pulls people towards cities and not sparsely populated rural areas.