Most stamp collectors who collected in the 1950s and 1960s maintained a collection of plate blocks and First day Covers along with their regular collection of US mint. There were two reason for this. First, a plate block was only four stamps and the face value of putting away a new plate block was 16c or 20 c, a modest amount. The same could be said for a First day Cover, the stamp was inexpensive and a nice Artcraft cacheted cover to make the First Day could be had for a dime. Both FDCs and plate block collecting were inexpensive adjuncts to collecting US stamps.
This has changed considerably over the last fifty years, and the trend has accelerated in the last twenty. First Day cover and plate block collecting have waned, and the reason is more than the fact, that the postage prices have driven up the cost of plate and FDCs. In the 1950s and 1960s, the US Post Office issued ten or fifteen stamps a year with a total postage value of under a dollar. Now, in most years there are well over a hundred stamps. For most stamp collectors, philately is a casual hobby, one on which they happily spend a few hours a month. Then they put their albums back up on their shelves. Obviously, in the 1960, the average of one 5c stamp per month that was being issued didn’t fill the collecting needs of the average casual philatelist. He could put that stamp away in his album in a few minutes. Plate block and First day collecting was born and thrived when the number of newer US postal issues was low, and collectors wanted other allied items, to spend their collecting time and money on.
Today, collecting has largely changed, since collectors have hundreds of newer issues to choose from-enough to satisfy the casual collector. Accordingly now only the most avid US collectors seek plate blocks and FDCs along with their regular US singles. There were never an enormous number of the most avid collectors in our hobby anyway and when casual collectors no longer pursue these adjuncts to mainstream US collecting, popularity and prices have dropped.