This is a very general question, but I'll try to answer it.

It was largely a combination of massive resources, a large population, geographical location, and good timing. The area that is now America is massive, and has incredible resources, with a good supply of almost every resource needed. That doesn't mean that the US is entirely self sufficient, just that they have less of a need to import as other countries and they have the capacity to export a great deal. Domestically, the US has great rivers. Relatively smooth and bereft of waterfalls for the most part, they allow domestic trade. The fact that most of the territory that would become America was weakly held by colonial powers or that they were willing to sell(think the Louisiana Purchase, Florida, Alaska, etc.), made it so that America's expansion was largely lacking in major conflicts, apart from with natives. Regions like the South and its use in cotton and tobacco helped make America rich, and regions like the Midwest because a bread basket. In addition, America's isolation from most nations makes it virtually impossible to invade by sea, and the only nations that can invade it by land are Mexico and Canada, which have historically been weaker. One instance of good fortune came in WW1, as the major European powers, France, Germany, or England, had taken massive damage to either infrastructure, population, and both. The US's comparatively lower casualties and lack of damage to its country led to them being much less damaged, and put them in a position for rapid economic growth relatively to the other powers. You see this on a much larger scale in WW2. The US mainland remained safe and casualties were much lower relatively than other major nations. As such, the US became the post war economic powerhouse, and with the collapse of the USSR, the most powerful nation in the world