When I was attending art school in Detroit, an art direction instructor explained to the class that the reason that spokespeople in used-car ads "yelled" at the camera was that the commercial makers believed that in order to "break-in" to the family room with your sales pitch, you had to talk to urban, African-American families the way that they talked to each other, by yelling at them.
This may or may not be a bit of apocryphal racism. My girlfriend, also a native of Detroit and film director points out that the ads were shot outdoors in close proximity to traffic using onboard mics.
Here is a classic example featuring Mel Farr "Superstar". The pitch isn't nearly as loud and aggressive as I remember them.