Los Angeles is not a Mafia powerhouse for a number of reasons. First and foremost is that the Italian population here was traditionally minimal. Second, the ethnic white European populations (Irish, Polish, etc.), which traditionally supports Mafia enterprises (gambling, loansharking, unions, etc.) were never plentiful in L.A. either.
This means that the markets were much smaller for Mafia type enterprise.

Third, the sheer geographic size of Los Angeles and the expansive neighborhoods made Mafia style takeover near impossible.

Los Angeles was home to many white collar Mafia enterprises such as film and labor union racketeering. Many of the studios were founded and/or managed by men connected to the East Coast families. Read the book Supermob. It discusses the total domination of the film studios by associates of the Chicago Outfit until about the late 1980s. The Gambino and Colombo Families dominated the pornography industry in the same way until about the late 1990s (with the rise of the internet).