Why did missionaries like marcus and narcissa whitman and henry and eliza spalding move west

To teach Native Americans about the Bible and Christianity

Marcus and Narcissa Whitman were among the early settlers of the West, pioneers of the Oregon Trail.  Their missionary party, headed to Oregon in 1836, included also Henry and Eliza Spalding.  The two wives were the first white women to cross the Rocky Mountains.  Over time, the Whitmans' work in the West contributed more to white settlement in the region than it did to the betterment of the Native America peoples they sought to work with.   The Whitmans and a dozen other white settlers were killed by some of the Cayuse people in 1847 in what became known as the Whitman massacre. The Spaldings were not among those killed in that event.  Henry Spalding continued to work among native tribes in the West. 

The answer is crop yield.