Any major change is the result of many factors, and most of these have been discussed here. I would add the development of container shipping. The container is an easier and quicker way to move freight the last mile from railroad yard to factory/warehouse/retailer. Think about the time and effort needed to move a box car from the Oakland yard to South San Francisco to a siding at a Third Street industry versus the time and effort to move a container from Oakland to the Third Street industry.