Postal stationary has always languished in popularity. In the earliest years of our hobby most albums had spaces for Cut Squares (the stamp portion of a mint entire envelope cut away from the envelope and collected on its own). But with the exception of US and German Area philately there are few postal stationery collectors anymore. This is largely because there are more than enough stamps to collect and collectors typically like material that is easy to find and readily salable. As the number of the world's unique major Scott listed stamps has approached one million, the feeling of most collectors is that they don't need to go far from traditional stamps in order to find lots of material to collect.
Another factor that negatively impacts the popularity of US postal stationary is how very difficult the catalog listings are and how trivial are the differences that make for major numbers. As a rule collectors are more drawn to face different varieties and catalog makers do a better job in popularizing specialties if they make minor design types into "a" numbers and not major numbers. Too many distinctions between designs of the same color and denomination are not appealing. Accordingly, postal stationary collecting is becoming even less popular. There seem to very few younger collectors of this material and US cut squares and entires are far less popular now than they were thirty years ago.