There is one key difference between the Aurigan Coalition and all those other micro-states (even the Micans iirc, but they're in only one system so it's moot):
The Aurigans have no JumpShips of their own. They completely rely on the Gwendor-Dorwinion Cartel. Their corner of the universe is explicitly called "JumpShip starved".
Planetary populations aren't that relevant for an interstellar nation, but without JumpShips you can't have an interstellar nation. All the nice stuff on Mechdur is worth diddly squat on Coromodir without JumpShips.
(There's a reason the Coalition only came into its own when Gwendor-Dorwinion showed up. And maybe the war ravaged the Coalition so much that the cartel's JumpShips moved to greener pastures elsewhere.)
Citation needed. Every corner of the universe is jumpship starved. The Aurigans were still able to conduct an interstellar conflict with themselves involving significant numbers of troops (for a periphery state). Show me in the lore how the Aurigans depending on the goodwill of privately owned jumpships is definitively different from every other small state that's on the map.
If those jumpships then left, and the Aurigan Coalition collapsed as a result, then... the Coalition collapsed. The state's dead, what's your point? It solves the problem of them not being on recent maps by destroying them as a state.
that also help explain the lack of map appearance. with no comstar registered jumpships travelling around in that little cluster of stars, maybe comstar just literally has no stellar coordinates for the worlds there, and thus leaves it off the map.
If that were true, (which it isn't because comstar is actively involved on Aurigan worlds running their HPGs) it still wouldn't solve the problem of other lesser states that Comstar also doesn't have any interest in appearing on the map. You think Comstar was going out of its way to monitor the life and times of the Illyrian Palatinate? Only bandit states like the Marians or Ciricinans care what the Illyrians are doing. And that's just so they can steal their stuff.
(yes, I do keep switching which minor state I use as an example, because there's more than one really good example)
On the contrary. ComStar may be the most active user of JumpShips there, but apparently is unable to double as a JumpShip service (like they seem to do in the Inner Sphere, the "ComStar bus lines" theory) out here.
As Periphery goes, the Aurigan Reach has an unusually high concentration of HPGs, all still serviced by ComStar. There's one Explorer Corps HQ out there, too. One planet is considered inhabited solely because the Star League built a HPG there for some arcane reason which ComStar still mans. I believe there are even gaps in the net out here that would cut some HPGs off unless ComStar brings in mobile HPGs or courier JumpShips.
ComStar is definitely aware of the Coalition, but apparently don't consider them noteworthy, or maybe simply kept out of politics and didn't recognize them as a state for neutrality reasons. (Remember, most of the time ComStar actually is just what it says on the cover.)
They didn't appear on the maps because they hadn't been retconned into the universe yet. The question isn't why they didn't; it's whether or not they will in the future (including the future of the past), and if they don't, why they don't.
The reason I'm pushing the fallen state idea isn't because of any grudge against the Aurigans, it's because I think it's the most likely solution for the writers to give us. It's simple, it's not an unreasonable result from a lore standpoint, it takes another extraneous piece off of a very cluttered board, and it doesn't require the positively herculean mental gymnastics we're seeing here with people trying to convince themselves that the Aurigans remained in a viable state but were still somehow so unimportant that they got beat out by the Elysian Fields when it came time to make maps.
But hey, the writers gave us forever faithful, so who knows, maybe they'll just jam it in there and call it good.