The availability of open range land in the West: The vast majority of the land in the Western United States was still open range, meaning that it was not owned by anyone and could be used for grazing cattle. This made it possible for ranchers to raise large herds of cattle without having to buy or lease land.
The development of the railroad: The construction of the transcontinental railroad in 1869 made it possible to transport cattle from the West to the East quickly and cheaply. This opened up a huge new market for Western ranchers and made cattle drives a viable business.
The invention of the six-shooter: The six-shooter revolver, invented in 1836, gave cowboys a powerful tool for defending themselves against predators and outlaws. This made it possible for ranchers to drive their cattle across open range without fear of attack.