The trusts became dominant in American industry because of the then practice of laissez faire capitalism: the belief that government should not involve itself in business. The result was a number of shady business practices and opportunistic gambles on business. Andrew Carnegie knew nothing of the steel business, but accumulated enough cash to purchase a steel company and then bought out his competitors one by one when business was bad. John D. Rockefeller actually ran competitors out of business. He formed a marketing company and dictated to the railroads the amount he would pay to ship his oil, and also spied on his competitors. In six weeks he managed to squeeze 90 per cent of his competitors out of business by selling oil below cost which they could not afford to do. When they finally collapsed, he bought them out at his own price--normally substantially below market price--and soon controlled over 90% of the U.S. oil refining business.
None of this would have happened in the present because of government scrutiny and anti-trust legislation which was not in existence at the time. Those with the necessary wherewithal soon learned how to take advantage of the weakness of competitors and thereby eliminate competition. When the competition was eliminated, they created a trust which operated virtually the entire industry.