Why did some Plateau peoples travel across the Rocky Mountains to hunt buffalo?

Plateau peoples traveled across the Rocky Mountains to hunt buffalo primarily as a result of the increased mobility and cultural shifts that followed the adoption of the horse in the early 18th century.312

The introduction of horse technology allowed some groups to transition from a lifestyle centered on permanent riverside villages to a more nomadic existence. By traversing the mountains, these groups could access the Plains to hunt buffalo, a practice that led them to adopt tepees and other cultural forms associated with the Plains tribes.12

In some instances, this movement was also influenced by external pressures; for example, around 1800, the Flathead and Kutenai were displaced from their home territories by the nomadic Blackfoot. After resettling in the intermontane valleys of the Rockies, these groups continued to make occasional buffalo hunting expeditions to the Plains, often in the company of other Plateau tribes such as the Nez Percé and Coeur d’Alene.3