Why did he use "you was" not "you were"?
"You was" here is grammatically incorrect and not standard English. However, in some parts of North America, people speak like this colloquially. It is less common today, but back when the movie in this video clip was around, lots of people spoke with incorrect grammar. This kind of speaking often could reveal someone's social class. People who were less educated or didn't have access to education grew up speaking very colloquially and using a lot of slang. Today, people still speak like this in certain parts of the United States but it is not as prevalent as before.