At the beginning of the fifteenth century, the world was a fairly small and contained place for Europeans. While they knew about far-flung areas such as China and southern Africa, their world view was still narrowly focused on Europe and the Mediterranean. Within two hundred years, Europeans would be all over the world with settlements on every continent except Australia and Antarctica. By 1600, most of the coastline of the Americas would be under the domination of Europeans as well as all the major cities in eastern Africa. The simplest and most obvious answer is the growth of mercantilism in the high middle ages.
Mercantilism is a simple economic activity.
All it involves is the purchase of certain goods in a region where those goods are common, moving those goods to another region where they aren't common, and then selling them at a profit. As simple as this sounds, the European economy was only minimally mercantile through the early middle ages. One reason was the relative scarcity of money. Once Europeans learned mercantilism, they set about it with great enthusiasm. The European economy quickly changed from a predominately barter economy to a predominately money economy, and goods from all over the world began to circulate throughout Europe.