OSRIC, it's activity levels, and its "promotion" really gets back to how what caps an era often results in a change into a different era, and the resulting near-obsolescence of the capping thing.
There are two drivers to the difference between OSRIC and later "OSR" rulesets:
1) apart from the rules themselves - all the then-true presumptions about the user base/market that OSRIC was built around
were themselves invalidated by OSRIC;
2) floaters want and need a central personality behind the game they're floating to and from. When someone says "LOTFP" who is the first person that springs to your mind? Now try "Swords and Wizardry"; "Old-School Essentials"; Astonishing Swordsmen and Sorcerers of Hyperboria"; "DCC" - notice a trend? There's almost always a creator's name linked to the game, both mentally and also in discussion. Game/creator; creator/game.
Now say "OSRIC". Notice what's different?
Now say "1st Edition AD&D"? Was that more like the first group, or more like OSRIC?
Apart from those 2 items very specific to OSRIC and the OSR as it happened, you then multiply all of that by generic dynamics that no game in the OSR is ever going to rise above:
A) how one makes themselves a name
B) how what made a name is nearly impossible to sustain
C) how the easiest short-term method of staving off the decline of a name creates its replacement
D) how each 4-year (give or take) cohort primarily wants to elevate something created in "their time"
E) how each cohort will then reasonably loyal to something created in their time, subject to the half-life rule
F) The saving grace that you only need 1,000 fans to make a living; i.e., even if noon doesn't last very long, sunset takes a long time to arrive
Each of the above in a bit more detail
1) OSRIC was born in a world where D&D was the BBC. It was not the cable TV world, or the streaming a la carte channel world; it was a world where everyone that later fragmented into very specific flavors was still largely united under the banner of AD&D as the Rosetta Stone of Not-3E play. If in 2015, you were someone who'd be touting Advanced Labyrinth Lord, what were you playing in 2005? AD&D. You might be playing it like ALL eventually codified, but you were "playing AD&D". What about someone who became an AS&SH aficionado? AD&D. Etc., etc. To the extent you read, wrote, and distributed fan-material in a world where there was no profit motive for your game due to fear of legal action, you did it either in the conventions of AD&D;
or in a grudging compromise of "old school feel/3E rules" such as Goodman and Necromancer Games.
It was just expected that everyone would run fan material written in the universally-familiar AD&D standard to taste. Or use the compromise commercial 3PP Necro/Goodman material while ditching the 3E rules they were written to both comply with and be easily back-convertible from.
But many individual specific tastes dipping into and drawing from very limited broadly compatible "channels" was how it had always been, and how it would always be.
Everyone making OSRIC could see the immediate attraction of a
replacement channel that was free of both the no-profit limitation of faithful content, and the no-faithfulness limitation of the for-profit content. This seemed to solve all problems, because people griped about both of those things, but no one was griping about using broadly-compatible AD&D formatted adventures for their narrower gaming tastes.
But shortly after publishing OSRIC it became clear the previous consumption behavior centered around AD&D content was based on the assumption that was pretty much the limits of the possible; what people are content with changes with what is possible at that time.
No one conceived that people
liked to buy rulebooks, for the same reason people
like to buy cars when the car they have still runs just fine. You go back and dig up old posts by people in both scenes over the entire period who now extol new rulesets, then saying one of the best things about TSR D&D is no one had to keep buying books!
No one considered the possibility people associate "living games" with "visible/active creator-owners". Everyone was playing a game that still (in 2006) had an active, living creator for two more years. That shortly this would be a vacuum, and the different ways that vacuum would be approached by different people, was not on the radar. This also ties into...
Matt was a guy who used AD&D to run OD&D without realizing he was mainly running OD&D. He thought he just liked a really light AD&D. But Jerry Mapes, the founder of K&KA, before he passed, asked Matt to look at doing the same thing for OD&D (Jerry's game of first-choice) that had been done for AD&D through OSRIC. That started a process that led (IIRC) to Matt realizing his heart was really in the creative, free-form possibilities of OD&D. And so you get Swords and Wizardry. This is where Matt's passion really was. But the simple reality is Matt was the guy who made OSRIC possible. OSRIC was a bunch of guys with a bunch of deep AD&D-specific knowledge, interacting with Matt's drive to write (and his own knowledge also).
But Matt was the guy who wrote manuscripts to completion; P&P was the editor/project manager/secondary writer who drove projects to a finish line. Both had access to a lot of fact-checkers and idea-bouncers and process-experts who could infuse all of that to an even richer1E broth - the best sort of contributors, who excel at apologetics and essays. But that talent doesn't necessarily require enjoying writing playable content, let alone prolifically writing playable content.
So what happens when the manuscript-writing guy finds his gaming heart somewhere else? The bottom layer drops out of everything. OSRIC lost half of its public face, and also, critically, the guy who had the time (being retired) and energy to write the most adventures. At first this wasn't expected to be any big deal because the thought was still that there was a tremendous pool of independent content creators who'd want to write OSRIC-AD&D adventures. It wasn't thought that a game needed a face. Gary was still with us; he was the face of old D&D. And then that suddenly changed, leaving a tremendous vacuum that (I believe) fans of that style instinctively wanted filled. Which leads us into #2