I've seen this across collecting communities - not long ago, I sold off most of what remained of my vintage racing bicycle collection - a lot of Eddy Merckx through Greg Lemond era stuff. I had to take peanuts for things like a Masi frame that would have been swarmed at the same swap meet 10 years ago.
That being said, I wonder whether some of the issue with long rifles has to do with the fact that many of the nicer examples would in the 1970's have been associated with the Revolutionary War, whereas dating now has pushed them much later. So they have lost their association with Bunker Hill and Brandywine and Cowpens. Instead, they are from an era of history most Americans know very little about, until suddenly the Civil War happens. I think some of that appeal to the imagination has gone once you lose those iconic names and places from the pages of 5th grade history books. This is me spit-balling - I'm here to learn.
I think a second aspect of this has to do with social media and the rise of platforms like YouTube. The popular "guntubers" that feature historical firearms - for example Forgotten Weapons - rarely if ever touch on American long rifles, in part because the knowledgeable people on those channels didn't grow up with an interest in firearms from the 18th and early 19th centuries. They really only go back as far as maybe the Civil War for the most part. So younger would-be collectors are being fed a steady diet of WWI and later firearms associated with major conflicts, or perhaps interesting guns from the period of rapid innovation in the mid-late 1800s, compared to which the technology of long rifles is quite static for the most part, and not all that interesting in and of itself. In the world of YouTube, I think that technology factor is huge.
Thirdly, people who can afford the best of the best and know what they are looking at, and who are on the hunt, can be very stingy with their knowledge, for obvious reasons. So it stays trapped in the heads of a previous generation of collectors. Yes, there are the books we all know, but the real knowledge takes time to acquire and refine. It's easier to watch a YouTube video or a "collectors guide" to know what to look for, and as we all know, a deeper appreciation required handling multiple examples and sitting at the feet of a master. It takes time, and we are not a patient society.
Fourth, I think long rifle collectors represent a unique confluence of people interested in firearms and people with an appreciation of aesthetics more generally. That is not to say aesthetics isn't a factor for many firearms enthusiasts, but when it comes to American long rifles, the aesthetics are primary. These cultural objects have more of an affinity with fine furniture or blown glass or a beautifully executed picture frame than they do with, say, a custom Colt 1911. And I think that culturally our aesthetic appreciation is in general rather poor. Look at the buildings and objects of use we are surrounded by every day, emphasizing utility over being pleasing to the eye.
These are just a couple of thoughts from a bottom-feeder in the arms collecting community prompted by this very interesting and informative discussion. Apologies for the 20-minute drum solo.