While I don't have the shooting experience of most on this board, I've been around bird hunting in the Northeast for over four decades in New York, Vermont and Maine. We commonly encountered other hunters and they typically shot SxS and O/U guns out in the field. Within the past decade or so I've seen many more pumps and semi-autos, or shiny new O/U guns. While it is good to see younger guys and gals participating, I agree that there seems to be much less appreciation for vintage guns, and many valid factors for that have been mentioned in this thread.
For me, there is a quality in these older guns, and Parkers in particular, that I have difficulty putting into words. I've referred to it as a "certain richness of character" that newer guns, even high-end guns, don't seem to have. I am always quick to say that this is my personal perception and I do not mean to insult anyone who has chosen a newer gun, or infer any less quality in their choice.
For some, they may just have never been around older guns and don't know about them. I frequently try to engage others in what these guns mean to me. Most are surprised that I know the month and year that the gun was made, where it was shipped, it's grade and options, type of Damascus, engravers by name, etc. I get odd looks when I say: "this gun is coming up on its 116th birthday next month." For many, they "just have a gun" and it meets their needs which is perfectly fine with me, but I take it as an indicator that there is decreasing appreciation for older guns.
One factor that has not been mentioned is that many opt for a single barrel or O/U gun because they don't like the SxS sight picture. Much of what you see in writing today seems to point towards a single barrel sight picture as an "easier" or "recommended" option for new shooters.
I shot my very first round of Sporting Clays two Sundays ago and it was a very interesting (and humbling!) experience. I group of guys agreed to take me with them to show me the course and they were very generous with coaching and calling my missed shots. I heard "behind on that one" and "stopped your swing again" a lot.
Another one I liked was "you knocked some dust of that one, but we don't count those."
There were at least 50 shooters there in all, and I was the only one with a SxS. By far, most were shooting new/newer custom O/U guns that were specifically designed for the sport. As a side note, almost all 12ga too; I saw only one other 20ga shooter. Only one older gentleman asked me about my Parker and his comment was that he did not like straight-grip guns. While my shooting was not on par with many that day (53/100), I had a blast and I am looking forward to many more rounds with my vintage guns.