I think that a variety of factors converged on the United States' economic stage in the 1990s. One of the most intense was the advance of technology. The advent of the internet and information technology on a mass scale enabled new revenue streams and new opportunities for businesses to emerge. The "dot.com" sector of the U.S. economy emerged and proliferated to a level that helped to spurn economic growth.
At the same time, the global marketplace began to take form with the collapse of the Soviet Union and the widening of communication advances. This helped to create new markets for businesses to develop and enhance revenue opportunities. The widening of the telecommunications market, in part due to the breakup of AT&T, helped to create more businesses and generate more wealth.
The signing of the NAFTA treaty also helped to create a more even flow of business traffic between the North American nations. Given the contraction that the economy faced at the end of the 1980s and into the early 1990s, the economic boom that followed it reflected growth and new opportunities emerging in different sectors of the economy.