The American press underwent big changes during the 1830s that led to the establishment of the mother of sensationalism, the penny press. Unlike their six-cent predecessors, these papers sold for merely a penny.
The New York Sun became the first penny paper and soon had largest circulation of any paper in the city.
News was no longer strictly for the elite. Middle-class Americans now had the opportunity to educate themselves about what was going on around them. Well, except politics. That kind of information was still nestled in the six-cent papers, out of reach for most Americans.
Unlike previous papers, the penny papers could afford to operate without help from political parties. Instead, they relied on heavy circulation and advertisements.
The shift in focus from a subscription-based (emphasis on the customer) business model to an advertisement-based (emphasis on the advertisers) business model led to the rise of sensationalism in America.