Why?

Because it is the freest period in the history of man. 

Was it a libertarian panacea? Nope. There were, in fact, infringements on liberty, such as the violation of women's rights, the Sherman Antitrust Act of 1870, compulsory school-attendance laws in Massachusetts, and others. 

But in terms of economic liberty, there is nothing that can match it. 

Imagine: 

No income taxation or IRS. People were free to keep everything they earned.

No welfare, including Social Security and Medicare. Charity was entirely voluntary.

No drug laws. People were free to consume, possess, or distribute whatever they wanted.

No immigration controls. Everyone was free to come to the United States.

No minimum-wage laws. 

Very few economic regulations. Economic enterprise was free of governmental control.