The most important difference was that the US was a nonthreatening backwater at the time. When France declared itself a Republic, every country in Europe and some parts of France itself declared war on it, so between the foreign and civil wars France was involved in, the country was fighting a war on twelve fronts. This led to the Terror, because from two directions the countries at war with France could get to Paris very easily (from the Vendee, Britain could get in, and the Vendee was making no secret that they were inviting French aristocracy who had fled to England in; and from Alsace-Lorraine), and people from there could easily came into Paris (as evidenced by the Girondine Charlotte Cordet coming and stabbing Marat).