Another important factor for any country to become a leading capitalist nation is its willingness to take risks. I think we must be one of the most motivated countries when it comes to investing. Americans love to track down their investments and we also love to move our "assets" from market to market and place to place depending on which way the tide is moving. I, personally, think that it takes money to make money. I think we are very good at taking intelligent risks with it. I may be wrong, but I stand by my view. :)
The efforts of the United States government to support businesses and encourage capitalism was a big reason for the early success of the United States. As mentioned by other posts above Japan and China have sped past us in world dominance as far as the business world is concerned.
I would also add that people in American were not afraid to work. They did what they needed to do in order to provide for their families, even if it meant work that may be considered demeaning. Today, I'm not so sure that is true. Americans argue that illegal aliens are taking jobs away from Americans, but many Americans are too "proud" to clean houses, pick oranges, work in a factory, or do other work which may be "beneath their status". They would rather complain about not having enough money or not being able to be gainfully employed than to lower their standards. This lack of work ethic will be a huge problem in the US if it isn't already.
What has made China become more capitalistic in recent times? And Japan? The United States became the greatest industrial nation at one time. Other countries needed its products, and the U.S. sold them, and sold them at a profit. True, the government was supportive of this industry, etc. and was organized for profit for capitalism.
I would support #3 as brettd makes a convincing argument. It is not just possession of land and natural resources that results in success and power, as Russia and vast numbers of African countries demonstrate. It is how they are managed, used, and exploited that results in success. This is something that the USA has done incredibly well.
I would add that a government system that has always been very friendly to capitalist business practices was even more important than space or resources. Take Russia for example, with much more land and seemingly endless resources, yet they have not been able to develop them or achieve a sustainably decent standard of living because of, in part, a hostile government.
In the US, we refused to seriously regulate business or labor until the 20th century, and this gave rise to the billionaire robber baron Rockefeller, and others like Carnegie and JP Morgan.
There are many possible answers to this question. I will argue that geography has made the USA the leading capitalist country in the world.
America's geography is important in two ways. First, geography gives the United States huge amounts of natural resources and a large population. This has been vital in allowing the US to create an economy that has become the biggest in the world. Second, geography has kept the US safe from the ravages of war. Because the US was untouched (in terms of its industrial base and its population centers) by WWII, it was able to dominate the economy of the world after the war. It was during this time that the US became the leader of the capitalist world.
Thus, geography has given the US its edge. Geography has given the US the resources and the safety needed to bring it to the forefront of the capitalist world.