I think it is driven by labor productivity.  

In a world where chemical is sold by the truckload or pallet full or tote, less demand for the local retailer to hand jugs out the door.

In a world of semi trucks, less demand to have an elevator two miles away for wagons. 

So the retailer consolidates locations and covers a larger territory from each plant and builds larger and more efficient infrastructure at that location, because transportation is relatively cheap and good labor/management is relatively expensive.