I picked up a few porcelain outlet boxes a few years ago. My guess is that at the time it was cheaper to make porcelain than metal. Also, since many people were scared of this new fangled electricity, they probably would have felt safer with touching something non-conductive.
Square D started around 1905 as a merger between a fuse company and Detroit switch. In those days electrical equipment was almost all open knife blades. Detroit started to put their knife switches in enclosures thereby make them Safety Switches. They put an embossed letter D, in a square, on the front of their switches. After awhile people began asking for that 'Square D' switch, so the company name was changed to Square D.