The Progressive Era was a turning point in creating a more activist government because the rapid industrialization of the late 1800s had created various problems that (some people felt) needed fixing and because it had created a middle class with the means and the desire to push government to fix those problems.
In the late 1800s, the US became very industrialized. This brought with it a number of problems. It created a situation where there were massive companies that needed to cut costs and therefore paid workers very badly. Those same companies sometimes sold products that were not safe. They often helped to corrupt the government to ensure that politicians would not do anything to jeopardize their ability to make profits.
At the same time, the industrialization started to create a larger middle class. These were typically white collar workers needed for the huge new companies. The middle class came to feel that government needed to respond more to their needs. They felt the government was being corrupted by the rich and the big companies. They wanted it to do more to regulate those companies.
Thus, the industrialization of the US created a situation in which the Progressive Era could arise. The Progressives forced the government to take more action towards things like antitrust laws and protections for workers and consumers. This was the beginning of more activist government.
The Progressive era, which lasted from the 1890s to the 1920s, gained steam due to the financial panic of the 1890s and ended at the close of World War I. During this time, the financial collapse made people open to new ideas, with many believing that a system that seemed primarily to benefit the so-called "robber baron" industrialists and corrupt politicians had to change. Immigration had skyrocketed, and the country, though still largely rural, was growing increasingly urban. The large pockets of people crowded into cities made government-level action more imperative: people simply could not solve problems created by industrialism on their own.
Improvements in technology made it easier to take indoor photographs in poor light. This made it possible for a journalist like Jacob Riis to document the terrible living conditions of the poor in urban tenements, leading to an outcry, and the impetus for government action. Upton Sinclair's The Jungle, though fictional, described appalling filth in meat-packing plants, leading to a consensus that the government needed to take action to regulate the food industry. Likewise, concerns about the effects of untested drugs on the market led to for calls for reform. All of this led to the 1906 Pure Food and Drug Act.
During this period an income tax and many reforms were enacted. While the federal government still played a completely minimal role in social welfare, the groundwork had been set for the New Deal of the 1930s.