Because Europeans wanted to use the resources of the new continent and eventually entered the conflict with the Natives. Conquistadores, as they were called were more advanced and therefore defeated and even destroyed some of the native tribes.
Explanation:
Wars that started led to destruction of some tribes, even certain civilizations.
On the other hand, another important element was the introduction of diseases by Europeans.
Namely, Europeans brought many diseases with them and the Native didn't have developed immune system.
