I'm a comparative latecomer to collecting U.S. revenues, only collecting them since about 2002. I missed the era when you could buy revenues by the pound or metric ton because nobody wanted them. There has been considerable increase in both interest and catalog values in U.S. revenues over the last 20 years, especially online.
While interest and membership in formal revenue organizations/clubs/collector groups has been on the wane, overall the interest from collectors is on the increase. As an
ebay seller, there is plenty of interest, and as both an
ebay buyer and at traditional auction, the competition has been quite fierce, even before the COVID price boosts.
As to what to attribute this to, who knows?
1. Collectors filling up their front-of-book collections as far as they can (whether by choice or finances), naturally then migrate to back of book sections.
2. Increased discretionary income in certain countries fosters hobbies and interests, hence more people in "developing" countries being interested in revenue stamps, not only in their own stamps, but in U.S. and other coutnries' material as well.
3. The Internet has exposed people to just how freaking cool revenue stamps and documents are, both aesthetically and technically (rates, usages, etc.)