The short answer to this is that the Columbian Exchange happened because Columbus “discovered” the New World.
“The Columbian Exchange” is the term that we use to refer to biological and cultural exchanges that happened between the New World and the Old World. These two land masses had been separated for thousands of years by the time that Columbus sailed in 1492. This meant that things were very different in the two worlds. There were different plants and animals in the two worlds. The people of the Old World had infectious diseases that were not present in the New World. When the Europeans reached the New World, this changed. They brought animals like horses, pigs, sheep and cows. They found plants like tobacco, tomatoes, and potatoes and brought them to Europe. Sadly for the native population of the Americas, the Europeans brought infectious diseases that killed (by some estimates) as many as 90% of the natives. All of this happened because Europeans came to the New World, allowing the flora, fauna, and cultures of the two worlds to mix in the Columbian Exchange.