Why did the tribes of Reuben and Gad ask to settle east of the Jordan?

Answer: The land suited their livestock

Reuben and Gad had very large herds and flocks and saw that Jazer and Gilead were suitable for livestock.

“Now the Reubenites and Gadites, who had very large herds and flocks, surveyed the lands of Jazer and Gilead, and they saw that the region was suitable for livestock.”— Numbers 32:1 (BSB)