Because that's a proportion with little market presence.
Most over 35s are happy playing the game they already have. They already have a favourite system and a shelf full of books to go with it. They don't particularly want to spend the time and effort to learn a new system. And this does double for anything outside core. I can imagine you buying the core three books of each new version of D&D. But I simply can't imagine you, with your stated tastes buying any source material in e.g. the Realms or Eberron. Or the Critical Role setting. You've got your own setting.
Perhaps,
But what will I consider buying, and not be alone in so doing? Simply put, anything that is or can be made system-agnostic. Adventure modules. Magic item and equipment guides. Setting material if it appeals to me (e.g. when the CR setting comes out I probably will give it a good long look).
Of course. One thing to remember about when it happened is that the least financially successful version of D&D was 2e, not 4e. It both got overtaken in sales by a rival (the World of Darkness) and was so unprofitable (unlike 4e which was raking in about six million dollars a year through DDI even after the launch of 5e) that it helped drive TSR to bankruptcy. WotC might have bought the biggest brand in tabletop roleplaying, but it had been an utter mess for years.
True, and though there were myriad reasons why TSR had gone belly-up let's leave those for another day.
People who'd stuck with 1e through 2e were people (like yourself) who'd probably stay with 1e going forward. They were happy with their game. And the 2e fans who bought everything would probably keep doing so as long as there weren't massive changes.
By and large yes, though (as you'll anecdotally see by looking around even just in here) many did jump from 1e-2e to 3e when it came out.
That said, and this has bugged me about WotC ever since they took over, why not market to all of us? Put out 3e (and later 4e and 5e) but keep supporting the older editions as well via conversion guides, edition-agnostic material and adventures (or more conversion guides!), and so forth.
When 3e came out they dropped the TSR editions like hot potatoes. Then, rather incredibly (and IMO rather foolishly) they even more blatantly did the same to 3e when they released 4e. They've done the same with 4e on releasing 5e but at least they weren't so up-front about it.
But the other huge issue WotC had was that in the late 90s D&D was the old person's RPG.
So market to the old people!
The popular RPG among teenagers and 20-somethings was Vampire: the Masquerade. WotC's two goals were to keep the 2e spenders (they spent money) and to win back the teenagers and 20-somethings (they spent money). They had data on what sold for 2e (player facing splatbooks) but needed more on what people were playing instead of D&D.
The splatbooks sold in large part because they and some settings were all TSR had out there to buy. Meanwhile those other games had new core books to sell.
In part they did that. I don't know if the over 35s cutoff came before or after an initial scan of the results.
This is a very good point, and something I'd never considered.
So now my questions are these: what would the results have looked like had all the data been included; and does the full data set still exist anywhere such that those results could be generated if they don't already exist?
That's ... not my experience. But I live in London, and people move around in London. A year or two is normal and people move away for jobs. Also a group I was part of that lasted 25 years had campaigns for a couple of years.
Yeah, London is a completely different environment than the small city I'm in.
And because 1e sets things up for the very long haul that way. Apocalypse World sets itself up for 6-12 session campaign - with significant character arcs in that time and possibly rewriting the world (which was created fresh and collaboratively for the campaign). In Pathfinder it took us about a year to get up to a level in the mid teens.
I'd just barely be getting settled in to a 6-12 session campaign before it'd be over.
PF has lightning-fast level advancement by my standards, as do 3e-4e-5e. I'd spend half my time trying to figure out what my character could do, just get it sorted, then have to do it all again because I'd bumped.
Financially if you are still playing 1e you are. It's a game that has been basically out of print for 30 years (yes I know about the recent deluxe editions). WotC is a business.
Out of print for 30 years now, sure, but at the time of the survey it had been OOP for less than ten years and still had a fairly strong player base.
Financially, if I'm a company I want to market to everyone I can, not just to a select group.