> The limited availability of lumber and the rapid pace of housing construction made other methods like concrete-block housing viable. This pressured further compromise because thinner 2x4s were a way to compete in price with wood alternatives. Size standards, maximum moisture content, and nomenclature were agreed upon only as recently as 1964. The nominal 2x4 thus became the actual 1½ x 3½, imperceptibly, a fraction of an inch at a time.

http://www.harvarddesignmagazine.org/issues/45/nominal-versu...

So it seems the reduction was was driven by cost saving, not a product of the finishing process, and it's actually only been standardized that way for 58 years.