What's with the southern accents?

Maybe I missed something or this is part of some obscure little known fact about California history and sociology; but, I was left scratching my head a bit on why so many of the white people depicted in this movie had southern accents. Is it because there is or was an actual subset culture of whites who had immigrated from the south in California at the time? Or is it because Hollywood thinks it is easier to play off racists with a southern accent? I seriously would like answers and/or opinions on this. As a southerner, I am always amazed at how the majority of white racists/sexists/etc. in movies seem to have a southern accent.