Because it is new. Maybe not new in age, but new in being a big city. Founded in 1800s, became second biggest city in 1980, third biggest in 1960, fourth in 1950: http://www.discoverlosangeles.com/blog/historical-timeline-l...

Big cities on the east coast are older, so didn't develop in the age of the automobile and have to conform to its needs. Except cities in the SE, which again, grew due to modern appliances like the air conditioner.