It's a case of corporate inertia.
Basically, years and years ago, Bud Orr made an aluminum body to take the guts from a PGP. This was so he could fit the marker with a vertical ASA, give it a removable barrel, and set it up with direct feed. Voila`!
The Sniper was born.
It proved to be very popular in part because
it had a removable barrel- few markers of the day did. It's rumored that Bud went with the barrel thread he did, because that's what at least one other airsmith had been using for his 007 barrel extension installs, but that's just a rumor.
In my opinion, Bud probably just found whatever off-the-shelf tap was available, that had a major diameter comfortably under 1", and a minor diameter comfortably larger than .695". That turned out to be the now-common 15/16"-20.
It's a nonstandard, but not unique or obscure size. A coarser thread like a more common 7/8"-14 would have had a minor diameter (the bottom of the thread grooves) too close to, or actually smaller than, the bore.
Anyway, that's what he used, and the Sniper took off as a very popular and prolific marker. A few years later, of course, the Sniper got converted to the AutoCocker, which had it's teething issues early on, but by virtue of it being one of only three or four semiautos in total
that were even available around that time, and due to the small but growing aftermarket support already surrounding the Sniper, it, too, took off.
The AutoCocker, of course, needed
modification right out of the box for those first few years, and because of it, it became the sport's Harley-Davidson, able to be endlessly customized and modified. The slab-sided body begged to be milled, and being aluminum, it could be color-anodized, unlike the then-direct-competitor AutoMag.
So a big cottage industry sprang up around the 'Cocker. Anyone with a drill press and a bit of skill was cranking out bolts and stocks, and everyone else was scouring army surplus bins for quick-pull pins and air fittings.
One of the things that took a little while to catch up, though, was barrels. Not everyone had the capacity to make a barrel, so for the first few years you were stuck with the stock tube (which you flex-honed for "better accuracy) or a longer version of the same thing. The barrel might have been removable, but there wasn't much out there to replace it with.
Then Smart Parts hit the scene. Their first product was the in/famous "rifled" barrel- early on, they basically bought supplies of stock barrels from the various manufacturers, drilled a series of spiral holes in the front half of it, and turned around and resold them at twice or three times the cost of new.
Again, thanks to the fact that literally nothing else was available, and to SP's now-famous shameless "tell 'em anything, they'll buy it!" marketing, sales of the "rifled" barrels took off like a rocket.
(As an aside, rumor has it the spiral drilling was actually developed by one of the early All-American team members, not
one of the Gardners. The guy did it in an effort to try and make a "self squeeging" barrel- the holes gave the paint somewhere to go as the next ball fired supposedly squeej'ed the tube clean. Didn't work that way, of course, but as we all know, SP never let a little thing like functionality get in the way of a good marketing campaign.
Anyway, those SP barrels launched the aftermarket barrel concept. Find some tubing about the right size, drill some vents in it, thread it for the most popular guns at the time ('Cocker, 'Mag, VM... that was pretty much it) and sell 'em for $80 a pop.
As barrel making proved to be lucrative, of course the makers branched out into the other markers- Illustrator, Pro-Am, etc.- but all the while, the 'Cocker was outselling them all by significant percentages.
So by about '94 or '95, there were half a dozen barrel makers, each making anywhere from one to five different kinds of barrels for the 'Cocker. It started getting to the point where, if you were about to develop and release a new gun, rather than coming up with your own new, unique thread, why not take advantage of the piles of existing barrels?
Enter Bob Long. He'd been a 'Cocker shooter for many years by this point, and so when he brought out the Intimidator (around '98 or so, as I recall) he threaded it for 'Cocker barrels, and that became a sales point. Already have a 'Cocker? Step up to a modern electronic marker and use some of the barrels you already have!
The Intimidator led to the Defiant (a 'Cocker-threaded version of the Bushmaster 2000), then the Tribal hit the market, then the Excalibur and Viking, the X-Mag, etc. and so on. Electros were the hot property, and a fair percentage of them were coming out of the box ready to take already-popular 'Cocker-thread barrels.
Which turns it into a self-perpetuating cycle. 'Cocker thread markers are some of the most popular out there, and they were made to take those barrels since there was already a large supply of them. Since most markers take 'em, any barrel maker that wants to stay in business will make them to match.
It is now essentially too late to change. Various manufacturers have tried ( Angel, Shocker, Ion threads...) but at this point, most of the more popular markers out there take 'Cocker barrels, there's a huge stash of them already in players' hands, and any manufacturer faces an uphill battle to try and change it. Their barrel thread may be better, but then the kids won't buy the gun because they can't get aftermarket barrels, or use the collection they already have.
If we had the chance to do it over, sure, something like Impulse/Ion threading might be a better choice- it's faster to unscrew, and being coarser, it's harder to strip- but that'd require a dozen manufacturers to collude to make the change, and a dozen more barrel makers to follow right along. And then tens of millions of players would have to decide that their $500 collection of carefully-selected barrels is junk, and would now not mind buying a handful of shiny new barrels.
So basically, no one person- except, really, Bud himself, and requirements were very
different back then- intentionally decided to go with that particular threading. It just worked out that way.