Two reasons that settlers traveled so far west rather than settling in places like the Great Plains, were legal and environmental.
The United States had treaties with several of the Plains Native American tribes, among them the Cherokee, who were driven out of eastern lands in the 1820s and 1830s. Oregon, however, was unbound by those treaties and thus "free" for settlement.
Several laws upended this though. The Indian Appropriations Act of 1851 started the reservation system. It was followed by a few other similarly named Acts that further restricted Native American territory and opening up the Plains to white settlers. The Homestead Act of 1862 was the law that really pushed the settlement of the Great Plains, with nearly free land given to whomever would develop that land for a certain period of time, among other requirements. Post-Civil War, hundreds of millions of acres ended up being claimed by homesteaders.
However, early explorers of the Great Plains, such as Zebulon Pike, termed it "the Great American Desert". While not desert in the way the American Southwest is, it receives much less annual rainfall than the eastern United States. Farming was difficult early on and even building structures was a struggle given the lack of timber. Most of the Plains was more suitable for grazing than farming. Conversely, the Willamette Valley in Oregon was extremely fertile with abundant water sources and timber for building.
It was the building of the railroads that really populated the Great Plains by white settlers. With rail transport, supplies could be shipped in and agricultural products (the wheat they got to grow there and cattle) could get shipped out.
Sources:
http://plainshumanities.unl.edu/encyclopedia/doc/egp.tra.028
https://www.nps.gov/home/learn/historyculture/native-americans-and-the-homestead-act.htm