I know there are Mafia families in other cities, but why aren't (or rather, weren't) they as widespread and influential as they are in New York and New Jersey?
Well, for starters the mafia families in Miami always had a connection to the Northeasterners. The Gambizi family actually had Carlo stay in Miami for a while after federal inquiries. Interestingly, the mafia settled in galveston island under Al Capone, who had a profound relationship with Genovese crime family of NYC. Galvestin was seen as a perfect refuge from the Feds- eventually, the Italian mob made its way to Cuba, who took advantage of the island's casino and resort repuation under Batista. After the 59 revolution, they founded Las Vegas (as we know it) and heavily influenced Hollywood.
The Los Angeles mafia had earned the nick-name "Mickey Mouse Mafia" by the LAPD. By 1988 the police had broken the mafia's presence in Los Angeles.
http://articles.latimes.com/1988-03-30/local/me-160_1_mickey-mouse-mafia