trais
Arcane
Aiur is in the Koprulu sector according to the wiki. It's weird but according to lore basically the entire strength of the Protoss and Zerg race is contained in a relatively small section of the galaxy fighting a small fringe element of humanity on the other side of the galaxy from Earth. At least according to lore the high protoss were always around Aiur and the majority of the Zerg swarm were concentrated to invade Aiur. At most the forces of Protoss and Zerg significantly outside the Koprulu sector amount to some Dark Templar and some scattered Cerebrates who broke free from the Overmind.
It very well might be, but that just indicate that Koprulu Sector is a rather huge swath of space, if Zerg weren't able to find Auir despied looking for it for how long exactly? Millenia? But it also in no way contradicts the fact that Overmind wasn't interested in expansion in Koprulu for the sake of expansion itself, but rather singularly focused on planting its spiky butt on Aiur. Because Xel'naga or something, something.
I can also buy that Humanity as a whole is way bigger and more powerful than Protoss and Zerg combined. After all, Khalai Protoss are the old, stagnant isolationist empire with no natural rivals to challenge them and spur them into action. Other than the Zerg of course, but they managed to keep them in check up until events of the game. Or at least they were convinced they were keeping them in check. E.g. Those walls of text between missions in Terran SC campaign, were describing how protoss fleet was cruising around Terran colonies, glassing every planet that got infested with the Zerg. Which could explain why Zerg wasn't overrunning the whole sector either, with possibility of Overmind simply wanting to keep Zerg real strength and numbers as a surprise.
And with Protoss being high tech race with advanced AI and robotic constructs doing any heavy lifting for them, I can see how they might had, at some point in the past, decided not to bother anymore with all this "colonizing other planets" nonsense, which Terrans are so universally fond of.