Collectors of United States stamps in the 1920-1950 period had a dilemma. The days of varieties were gone. The papers and printings of the Bank Note issues which span over 80 major Scott numbers were over as were the wild early days of the Washington-Franklins that spawned nearly 200 major numbers. Stamp issuing policy was conservative and the definitive set of the period-the Presidentials- had few varieties. Stamp collectors need new issues and less expensive stamps to add to their collection or they lose interest. So American collectors gravitated to plate blocks. It’s not that other countries lacked marginal markings-both German and British stamps have plate numbers and French stamps have marginal numbers called millesimes- it is just that plate block collecting caught on in the United States to satisfy this desire for more newer issues to collect.
The reason why Americans esteem their marginal markings more than other countries’ philatelists is simple. In Britain the desire for varieties of newer stamps was fulfilled by Gibbons proclivity for recognizing (and giving major catalog numbers to) scores of shades especially of the definitive issues of 1920-1930. And British collectors had the colonial issues. So did the French and the Germans has the older States and the Michel crazed minutia of the Weimar Republic (in fact there is no country catalog is more specialized than Michel). When the stamp issuing policy of the United States became more robust after 1980,  plate block collecting began to wane. Collectors could get enough newer cheap issues without resorting to plate blocks.
