The livestock owned by the Reubenites and the Gadites were unusually vast and numerous. They saw that the land of Jazer and the land of Gilead were exactly the place for livestock.
References for Numbers 32:1
So the Gadites and the Reubenites came and said to Moses, Eleazar the priest, and the chiefs of the community:
"Ataroth, Dibon, Jazer, Nimrah, Heshbon, Elealeh, Sebam, Nebo, and Beon is
the land that the LORD struck down before the Israelite community. It is a land for livestock, and your servants have livestock."
They said, "If you approve our request, give this land to your servants as property. Don't make us cross the Jordan."
Moses said to the Gadites and the Reubenites, "Should your brothers go to war, while you stay here?
Why would you destroy the Israelites' resolve to cross into the land that the LORD gave them?
Your ancestors did this, when I sent them from Kadesh-barnea to inspect the land.
They went up to the Cluster ravine, saw the land, and destroyed the Israelites' resolve to enter the land that the LORD had given them.
The LORD became angry on that day and promised,
‘None of the persons that went up from Egypt, those 20 years old and above, will see the fertile ground that I promised to Abraham, Isaac, and Jacob, because they didn't remain true to me,
except Caleb, Jephunneh the Kenizzite's son, and Joshua, Nun's son, because they remained true to the LORD.'
The LORD became angry with the Israelites and made them wander in the desert for forty years until the entire generation had died, which had done evil in the LORD's eyes.
Now you've taken the place of your ancestors, a group of sinful men, to intensify the LORD's anger against Israel.
If you turn away from him, he will turn away again to abandon Israel in the desert. Then you will destroy this entire people."