What main event prompted the federal government to take a stronger role in the economy

The answer is the 2009 Global Recession and the Great Depression.
For recent years, Before 2009, the United Stated was home to one of the freest business sectors in the world.

Years of deregulation had given rise to powerful companies and the financial sector, headquartered in Wall Street, was functioning globally with very little government insight.

The United States had become a complete laissez-faire economy where capitalism was going to allocate resources efficiently.

However, the 2009 recession occurred, in part, due to massive deregulation which lead businesses to carry out questionable activities with impunity.

After the recession, the government start to gradually play a larger role in how the economy was going to be managed.

Before the recession, the Great Depression of the 1920s saw the United States economy plunge drastically. 

Even at the time, the government had taken control of large parts of the economy in a bit to stir it back into growth.

Multiplying frequency by period