Nature abhors a vacuum.
But a glance at either of the "Cryptic Alliances as States" maps out there shows a large amount of uncontrolled space.
How to explain that?
Here are some ideas.
Populations are Small. Let me repeat this. Populations are small. Big cities in GW (at least as of the early editions) top out around 50,000 or so, probably with a nearby robotic farm (or, heh, Gallus Gallus 5/13 run chicken farm) to support them. In the modern world 50K isn't a city, it's barely a large town. There just isn't a surplus population to fill everything. Plus, the need for security in numbers means that people can't just spread out willy-nilly. What scant population there is clusters together in small groups up to little cities, and few of those. There is a vacuum of controlled area because there isn't enough population to control it all. Better to hold a smaller area security than to spread out beyond your ability to project real control.
Radioactive Badlands. There are a lot of these on the published maps. 170 or so years on from the disaster, there are still places with radiation lethal enough to mutate, or kill. Or both, one after the other. While probably not terribly realistic, that's not an issue in Gamma World. Radiation warps and grants powers good and bad, it doesn't just give you cancer and lesions and death. And it lasts a long time, again, unlike reality. I was at the hypocenter of Nagasaki the other day, my second visit to that atomic blast site, and was granted few if any cool mutations a mere 70 years on.
Yet they are there, and in swaths large and small. They significantly reduce the amount of safe areas to expand into.
Enemy Populations. Plenty of areas will be covered with vegetation that actively wants you dead. Some will be filled with lethal mutant animals. That leaves aside crazed humans and humanoid mutants. A wide area of lethal grass or trees is enough to make a land worth ignoring until everything else is taken.
Lands Gone Bad. Lands turned marshy, turned into crater lakes, strewn with rocks churned up into a rocky non-arable badlands, and so on. Lands gone desert-like thanks to previous chemical, biological, and nuclear weaponry.
Forbidden Zones. These places will be smaller but exist, often as a mix of the other types. They might be places you could go, but the locals, such as they are, don't want you to go there. These are off-limits only if you don't have the might to go there.
Surviving Installations. A few surviving installations are out there. They may be largely intact, partly intact, or mostly trashed. But if the defenses exist - patrolling warbots or death machines, security bots linked together, minefields, and so on - they will be impenetrable. A no-man's-land is likely to emerge around such places where it's too dangerous to live, and thus will lie fallow and unexploited. Few communities want to live just on the edge of a warbot's patrol zone. You are very safe until that day the bot decides you look dangerous and lasers you into an ash pile.
Deliberate Vacuum. Around the Cryptic Alliances, especially, you'd probably have a buffer zone. They leave it to nature to discourage approach without spending resources defending lands they can't fully secure and exploit.
On top of this, the land is essentially still at war at a low level. It's not peaceful - it's a largely uncivilized world. Remaining stocks of nasty, nasty weapons are used to settle conflicts, creating new badlands and new radiation zones. There is not a lot of produced tech (albeit more in a 2nd edition GW game than a 1st - 2nd assumes a lot of trade, linguistic standardization, and re-emerging tech.*) It's hard to fill all of the vacuums with civilization while populations are still recovering (and populations are a mix of mutant humans, humans, and mutant animals who may not be able to freely crossbreed.) Without a lot of tech it's hard to fully exploit what is out there.
So I'd expect a lot of choice lands (and easily exploitable installations) would be taken. The ones that are less than ideal would be left until the choice lands were fully taken, except by the occasional hardy adventurer turned freesteader. The lethality of the world would put paid to a lot of them.
So that's how I'd explain the vacuum. It's there because of the above and the inability to have enough people to exploit it. That's the disorder in this post-apocalypse setting, and it's the seam where adventure can happen.
* Stuff like the lexicon, the starting characters with needle guns with Intensity 17 poison needles, muskets and other black powder weapons in production, etc. are all clues about this. The pictures (especially the cover) for 1st edition imply a tech setting but the rules inside assume dirt-poor tech-ignorant barbarism.