This really depended on the time period.
Towards the beginning (just before the Gold Rush) and after the Lewis & Clark Expedition, the main appeal to moving west was the chance to get ~160 acres of land for free and buying the rest extremely cheaply. (so the first appeal was land).
The next appeal was gold, after gold was found in California and the surrounding lands, prompting thousands of gold-seekers to move out west to try to strike it rich, often risking their lives.
Another appeal was the appeal for work, as the Transcontinental railroad was soon being built, and large amounts of businesses were springing up, which allowed the economy to grow in the west, and spurred the advancement of western-US economy and society.
Finally, the appeal of a new life spurred Americans to travel west, as they wanted to restart their lives, and do something new.
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hope this helps
