The Gold Rush of 1849 caused thousands of people to flee the Eastern part of the United States and head west to seek easy money, dug up out of the ground. A lot of the lands Americans were infiltrating weren't being overseen by any kind of widely recognized law, and the boomtowns that hastily formed kind of had to make up their infrastructure as they went along. A whole new world formed: the Wild West. 

American writers of fiction were telling tales of the "Great American Frontier" as early as the 1820s (some bibliophiles may be familiar with the works of James Fenimore Cooper), but the Western literary genre began to explode in the late 1850s and 1860s. Pulp novels began to proliferate across the country, and the "penny dreadfuls" invented a lot of the icons and trappings we see in Westerns to this day: gunslingers, bounty hunters, lawmen, etc.