The USSR, the Union of Soviet Socialist Republics, was ostensibly a federal state. One of those states was the Russian Soviet Federal Socialist Republic. The RSFSR was part of the USSR, not its predecessor. Before the USSR there was Kerensky's provisional government, which proclaimed a Russian republic, and before that the Romanov monarchy.

I think a better way of approaching Soviet federalism is to ask why the leadership acknowledged Ukraine, Belarus, central Asian states, etc., as autonomous SSRs. Short answer: Lenin and the Bolsheviks were keen on federalism because they wanted to show their lack of national chauvinism. See Terry Martin's "Affirmative Action Empire" for a full treatment. If that's too much work, read the wikipedia page about Korenizatsiya.