Why did the Columbian Exchange happen?
Unfortunately, the Columian Exchange undoubtedly took much more from the New World than it brought and wiped out most indigenous populations.
Over the course of the 16th and early 17th centuries, the wealth taken from the Americas to Europe constituted one of the largest transfers of wealth (precious metals, gems, dyes, artifacts) in human history up to the present. And although Europeans brought many useful items to the New World, they also brought smallpox, measles, influenza and other diseases to a population that had no antibodies to resist such diseases. Historians and anthropologists estimate, for example, that this unintended consequence of the Columbian Exchange probably resulted in the death of between 70%-90% of the indigenous people in the Americas.
In addition, as European opportunists flooded into the Americas, some of whom started working the land and needed cheap (read, "free") labor, the slave economy became a central aspect of life for the remaining indigenous people in the Americas.
Just as the Columbian Exchange is seen as one of the most important events in history, then, it is also seen as the event that reduced Native American populations in the Americas almost to insignificance.
The Columbian Exchange happened because Christopher Columbus "discovered" the New World and other Europeans subsequently followed in his path.
The Columbian Exchange was the exchange of all sorts of things (including plants, animals, microbes, and even people) between the Old World and the New World. Since the two worlds had been largely separate from one another for millions of years, they had developed separate sets of flora and fauna. After Columbus came to the New World, things like horses, sugar cane, coffee, and smallpox started to flow to the New World. At the same time, things like tomatoes and potatoes went in the other direction.
So, the Columbian Exchange happened because, after Columbus's voyages, two "worlds" that had previously been separate came into contact with one another.