Why did the Gothic flourish in the USA in the early 19th century?
I think its important to pay attention to the changing attitudes towards religion at this time. Up to this point, religious beliefs dominated the public's understanding of human nature. God controlled if a person was good, or if a person was evil. There was no in between, and there were clearly laid out punishments for those that did not come down on the side of right. Predestination had taken away control.
However, as the Age of Reason came and went, views began to change. Science began to explore the nature of humanity and examine the effect of family life and environment on behavior. Predestination was abandoned as a doctrine in favor of good deeds and penance. In many ways, this was a scary way to contemplate the world. If no higher power was controlling our behavior, might we not all sink into evil? Gothic Literature explores this idea.
I'm not really aware what the official self-image of the United States was, but the reality was that the early United States Republic was place grappling with violence, poverty, racism, war, etc. In dark times, people have often taken solace in dark but stylized art forms such as Gothic. Truly depressing stuff portraying the reality of the young Republic's many problems probably wouldn't have been as popular, but in Gothic literature the reader can dive into darkness without being directly confronted with reality.
It was a rebellion to the restrictive ideas in the Age of Reason. Almost everything was the opposite...all reason, logic, society over the individual, restricted word use.