Because Germany had a long tradition of socialist politics and discourse extending as far back as Hegel. There was no way to avoid confronting this tradition if you studied at a German (or French) university in the early 60's: it was the platform for all social criticism. When the New Left proposed that Russian communism was a false development and not the inevitable result of a proletarian revolution or of socialist economic organization, it seemed to many that democratic socialism might yet be achieved by revolutionary means.
Because the generation conflict was far more pronounced in Germany, given that the fathers of many had been in the Nazi Party or were supporters. This was pretty much known to everybody, and really not so surprising, since Hitler had enjoyed massive support in the population, and where after all were these people supposed to go after 1945?
Because the murder of Benno Ohnsorg in June 1967 by the Berlin Police polarized the opposition community much earlier than the Kent State killings (May 1970) did in the US.
Because the Germans lacked a Summer of Love or a Woodstock to temper the vibes, and never succeeded in creating a more spiritually sustainable community until the creation of the Green Party in 1979 by Petra Kelly and others.
In America, despite the endless palaver about revolution, protesters acted without much thought of reorganizing society or restructuring existing political institutions. One went to anti-war demonstrations from time to time but worked more often to realize local interests -- civil rights, integration, respect for ethnic minorities, women’s liberation, gay liberation, anti-nuclear campaigns, etc. The anti-war movement targeted an inept and morally corrupt political policy, but few thought it necessary to re-write the U.S. Constitution because of it.