I think this is pretty much it. Everyone (1st appearance collectors, run collectors, and readers) were pretty much in the same market back in the day. Yes, some books were reprinted, but not all. If you wanted to read the Starline Silver Surfer run that led up to a huge crossover, you had to buy the comics. Now, readers aren't competing in the market for high-grade copies. And while books like X-Men 94 still appeals to run collectors, GSX #1 appeals to that market *and* first appearance collectors. Another factor perhaps in the X-Men situattion: when I started collecting comics in the late '80s, Claremont had been writing the book seemingly forever, and would continue to write it for a few more years. X-Men 94 (not Giant Size) was his debut on the title. (It's interesting how obvious they were trying to make X-Men the "international" team from reading GSX, where it wasn't blatant in Claremont's writing, IMO.) After Claremont left, I think the appeal of his first issue may have waned a bit.
Even if Silver Surfer 34 had far fewer copies printed than other big keys of the era, we're still talking about an era where a ton of copies were printed, so there are still plenty to be had, in a diminishing collector's market. I can say that when I sold my copies of Thanos Quest before Infinity War, I was happy with the prices.
Part of the thing might be that, even after stories became available to read in many other formats, older collectors have been conditioned to enjoy the hunt for books they needed to buy to read the story. Younger collectors never had that, so there's not that thrill for them of finding a book they need for the run (especially in the age of the internet).