The opening of frontiers always provides opportunity, as well as challenges. Many of the settlers moving into the area were looking for freedom to live their lives as they wished, which usually included the opportunity to gain land, which was the chief form of wealth in the 19th Century. Other people wanted to be able to tap into resources such as minerals in the form of iron, lead, silver and gold as well as animals such as beaver and bison. We also cannot forget the fortunes made in lumber from the old growth forest. A person who was willing to work hard could, possibly, earn a tremendous amount of wealth that those settled "back east" could not imagine.
At the same time, as these settlers moved west, they created another opportunity for merchants, doctors, teachers, and other "support workers". The miner needed somebody to produce food, tools, and all the other implements necessary to mine. Of course, the same is true for every other occupation. The true fortunes during the California Gold Rush, for example, were not made by the miners, but by the merchants selling things to the miners.
In summary, it was the unused or underused resources, either the products of the land or natural resources, that provided the economic opportunity, as it was almost like found money in that it was just "there", waiting to be developed.