The reason is that the American revolution wasn't viewed as a revolution. The American revolutionaries legitimised their actions by claiming that the England had encroached on their traditional rights as Englishmen. The monarch had acted as a tyrant and rebellion could therefore be defensible, if the aim was to restore their rights. The French revolution was a true revolution; society itself was transformed. The sovereign, the King ordained by God, was replaced by a new sovereign, the nation. The American revolution did not change the basis for society itself (atleast until the constitution was ratified); the French revolution overturned the ideology of the ancien régime itself, threatening every society in Europe. One way to exemplify this is to look at the foreign policy of the United States and revolutionary France. The former never tried to export its ideas, but many of the most outspoken proponents of the latter wanted to spread the ideas of the revolution throughout Europe by force.

I hope that makes sense.

Source: Max Edling, 2009: "Den konstitutionella revolutionens startskott", in Maktbalans och kontrollmakt, eds. M. Brundin & M. Isberg. Edling is the foremost Swedish expert on the American revolution and a researcher at King's College, London. His dissertation, "A Revolution in Favour of Government", is an eminently readable revisit of the American revolution from a state formation perspective.