I'm not the best on Social Studies.

( This helps with answering the Question): According to historian Norman Graebner, expansionists in the 1840's increasingly viewed Oregon and California as “two halves of a single ambition” to stretch the nation’s boundary to the Pacific Coast.

. . . With the Oregon treaty of 1846 the United States had reached the Pacific. Its frontage along the sea from 42° to Fuca Strait and Puget Sound fulfilled half the expansionist dream. On those shores the onward progress of the American pioneer would stop, but commercial expansionists looked beyond to the impetus [momentum] that the possession of Oregon would give to American trade in the Pacific. “Commercially,” predicted Benton [United States Senator Thomas Hart Benton from Missouri], “the advantages of Oregon will be great—far greater than any equal portion of the Atlantic States.” This Missourian believed that Oriental [Asian] markets and export items would better complement the mercantile [trade] requirements of the United States than would those of Europe. . . .

Source: Norman Graebner, Empire on the Pacific: A Study in American Continental Expansion, Ronald Press Co., 1955 (adapted)

(This is the Question) : According to Norman Graebner, what was one major reason for the expansion of the United States to the Pacific Coast in the 1840's?