I think that the best answer is the geographic location: many other countries (Sweden for example) have tried being neutral at different times but did not manage to: but the fact that the US has only two boundaries, and that both other countries also have a history of fighting from independence from a European power, makes it possible that unless US decideds to get involved in international politics, it can be "left alone".
It enabled Abraham Lincoln to issue the Emancipation Proclamation, by giving the Union enough confidence (The battle had forced the Confederate troops to withdraw, making it technically a Union victory) to change the focus of the war from bringing the wayward states of the South back into the fold, into a war to destroy slavery in America. One major consequence of the Emancipation Proclamation was that the European powers that were interested in helping the CSA were discouraged from interfering, given that their populations had a severe distaste of slavery, and a war to support slavery would not be taken well by their people.
