How and why was muckraking possible?

The history and rise of “muckraking” and the journalists who did it should be thought of as closely tied to the progressive era: the period from about 1890 to 1920 characterized by sharp criticism of political and business monopolies, social ills in the big city, and the dangers of industrialization. Before muckrakers, famous newspapers like those run by William Randoph Hearst and Joseph Pulitzer competed for readers by sensationalizing stories, and were focused more on riling up the public than on exposing any particular social ill--a practice called “yellow journalism.”
Muckrakers were aided by the rise of magazines like McClures and The Saturday Evening Post, and also by the advent of wire news services, which distributed breaking news more quickly and objectively. They thrived in a political atmosphere of reform, and the scandals they uncovered helped fuel political candidates across the country. Muckrakers were also certainly helped by events like the election of Theodore Roosevelt as president, who promised to break up monopolies and root out corruption--even though it was Roosevelt himself who coined the term “muckraker” out of frustration with reporting about his administration!

Muckraking should be viewed as a significant feature of the Progressive Era in the United States. During this period, myriad reforms, increased government regulation, environmental protection, and trust busting took place. The muckrakers informed the public of the issues of the day and helped win public support for significant changes. Rapid industrialization in the late nineteenth century had led to vast transformations in the country, and the muckrakers played an indispensable role in publicizing those that had had an adverse impact.
The term muckraker was first coined by President Teddy Roosevelt in a speech in April 1906. A muckraker, one who rakes muck, spread often scandalous news about shortcomings in the economy and society.
One prominent muckraker was Upton Sinclair, author of The Jungle. This book exposed unsanitary conditions in Chicago's meatpacking industry. Sinclair, a socialist, wrote the book in an effort to highlight the plight of poor immigrants and their long hours of drudgery in factories. The American public was incensed by the filthy methods and ingredients used in meat production and demanded reform. The result was the passage of the Food and Drug Act of 1906.
The case of Upton Sinclair was not an anomaly. Muckrakers often paved the way for political reforms and new legislation. Another example of this phenomenon was Ida Tarbell's scathing indictment of the oil business in her book The History of the Standard Oil Company (1904). Her book helped lead to the breakup of some monopolies.