The USA has been capitalist since it was a British colony, and Britain itself was capitalist. The United States inherited most of British cultural, economic, legal, and philosophical traditions.

It is less about "being sure that was going to work best" and more about respecting individual's right to own property. Property ownership is deeply rooted in British (and Western in general) culture, and was further developed by Enlightenment era philosophers such as John Locke, who contended that the right to own property is a natural right, along with a right to life and liberty. Capitalism naturally arises when individuals can own property and are free to use that property to conduct business.

Further, one can argue that capitalism is the "default" system that arises naturally without any government interference. For example, even in North Korea, which is obviously Communist and quite repressive, there is a significant black market. People naturally tend to buy and sell services from one another, start enterprises and make a living for themselves. And that is all that capitalism really is, just on a bigger scale.