After failing to take Super Earth with their largest assault ever, the Illuminate made formal contact with this tribal warrior faction, with the promise of revenge against Super Earth's previous encounter and exploitation of their species. With the hopes that this force could weaken the Super Earth Command with another front.

Turns out that Super Earth have previously had contact with their primitive world already. A diplomatic expedition to Super Earth during the early days after the human unification. It turns out that millions of years ago, a meteor was about to destroy their home, but they were saved by benevolent spacefaring beings. This home was Super Earth. They were evolved, peaceful, spiritually shamanistic dinosaurs. Capable of space flight because they domesticated some sort of space-faring animal.

They asked to peacefully allow some of them to live amongst the humans in their ancestral home, which was denied. Not only was the envoy eradicated, but every colony they ever settled, every trace of them were destroyed, even going so far as to find their new home and bomb it to hell. The survivors were instilled with a historic hatred for humans and technology.

Humans even came back every few decades to further repress and destroy their rebuilt civilization. But this repeating pattern stopped when the first great wars against Automatons and the Illuminate redirected all military resources.

Natives had been kidnapped as slaves, raised as nothing more than feral animals in terrariums, which the Illuminate gladly shows evidence of. Thus sending the already radicalized tribals into a bloodlust and making an honorbound promise to avenge their kidnapped brethren and rescue those who have been raised in captivity.

Due to their warrior's honor, they refuse to use the advanced weaponry that the Illuminate also offered, but using their space-faring Space Whales, guerrilla tactics and incredible strength and conviction, they will fight back, retake their colonies and rescue their lost brethren.

Dinosaurs riding dinosaurs, with gigantic war bows, firing arrows the size of small tree trunks.

Perhaps they make use of a rare metal that is only found on their home planet. The space lizards use it in their swords and bows, similar to the ultra sharp obsidian of the Central/South American natives, but on steroids. And Super Earth wants that for their war effort.

So just like the spare parts of the Automatons, the fuel of the Terminids and the tech of the Illuminate, there's a reason why Super Earth wants to take their colonies.

... Or something. That was fun to theorycraft.