The skip happened because the show was ending. They needed to tie up a lot of character arcs (Lucky's rise to power, Capone's arrest on tax evasion, etc.) and skipping years was the only way they were going to do it effectively without becoming completely haphazard.
Disagree. They don't have to include a full history of the beginning of the Mafia. There is no set "end" period. The show is about a mobster in Atlantic City, not about New York or Chicago. Sure, they figure big in the storylines and they are exciting to read about and watch (for me, personally) BUT they are secondary to the show.
