Because in the late 18th century, France was a vastly more relevant country than the United States. With a population of 29 million in 1789, this made it both the most populous country in Europe and also significantly more populous than the United States, which had a population of only ~4 million in the same period. France was the cultural hub of the Western world, and close rivals with the British in terms of scientific and technological developments. French was spoken by the upper classes of most European states, even as far away as Russia. It was French aid that led to the defeat of the British during the American Revolution. Put simply, France was a big deal. The cultural, scientific, technological and military ideas that dominated the era came mostly from them. Several Founding Fathers, most notably Thomas Jefferson and Benjamin Franklin, absolutely loved the French and considered them to be supremely important in world affairs.

Though the ideals of the American Revolution was universal, they did not in fact directly inspire many successors - aside from Haiti, there were no significant or successful revolutions between the period of 1775 and 1789. The American Revolution was very much a specific localised affair; it was a revolution led by Americans that benefited Americans. The Founding Fathers did not believe in actually spreading the revolution to other countries, ala the idea of a "perpetual revolution" that the French and later Soviets would adhere to. They'd solved their problem, and were content to mind their own business when it came to international affairs. This attitude of isolationism - shared by most Founding Fathers but confirmed as official policy by Washington - remained rock solid for a long time to come - even when fellow revolutionaries in France was asking for their support, the US refused. They did not believe that republicanism justified foreign wars. As such, the direct geopolitical impact of the American Revolution was quite limited compared to future revolutions.

France, on the other hand, was the polar opposite of all of this. When the monarchy was overthrown and a republic proclaimed in 1792, the revolutionaries immediately started expanding the scope of revolution. First the Low Countries (i.e. Belgium, Luxembourg and the Netherlands) were invaded and "liberated", being made into "sister republics"; then Italy came next. But the effects of the Revolution were not just limited to government structure; extremely progressive reforms such as complete secularisation and the abolition of slavery was a direct threat to the powered interests of other states, and wherever the large and skilled French armies marched, they brought their progressive reforms, toppling ancien regimes that had ruled for many centuries. Unlike the Americans, the French explicitly pushed for perpetual revolution; to liberate everyone they could even if it meant war. This was shown most clearly with the invasion of Egypt in 1798; though realpolitik men like Napoleon and Talleyrand pushed for the invasion solely for geopolitical purposes, many politicians - and especially common soldiers - viewed the invasion as genuinely being to liberate the oppressed Egyptian people from the outdated medieval rule enforced by the foreign Mamluks.

Moving past the early years of the Revolution, the effects of it continued to be directly felt for years; the revolutionaries in Latin America rebelled following the fall of the Spanish monarchy in 1808 and were led by men directly inspired by the French revolutionaries and to a lesser extent Napoleon (before his politics turned decidedly away from being left-wing). The Spring of Nations in 1848 was also directly inspired by the French Revolution of 1789, with several leading figures having been alive during that period. The guiding force behind both Italian and German unification in the middle of the 19th century was memories of the short-lived but progressive Napoleonic Code and other assorted Revolutionary reforms. Even in Britain there was more political instability due to the French Revolution than the American Revolution; King George III, who remained very popular and beloved by the British people during the latter event, had his carriage swarmed by mobs throwing rocks, and anti-government/anti-aristocratic rhetoric pervaded the press. For a while it seemed likely that even the ever-resilient British monarchy would be overthrown due to influence from the French revolutionaries (who also funded and supported a rebellion in Ireland in the same period).

The fundamental differences in their impact are found in the ideals pushed by both. The Americans' primary argument was for representative government; a rebellious but not excessively revolutionary attitude. The French, on the other hand, argued for the destruction of organised religion, the abolition of slavery, redistribution of land, and universal male suffrage. The American revolutionaries didn't go anywhere near this far. The revolution of the Americans was aimed against one specific government and did not spread far beyond that; the revolution of the French was against entire societal institutions and value systems and was intentionally spread to every country possible. This was understandably seen as both a greater threat and more significant historically speaking, as it proved to be.