The native people from the Americas went into a steep decline after they were colonized by Spaniards for several reasons. The first reason was the grave diminish of the native population in the Americas due to the viruses and diseases brought to America from Europe, like smallpox, that caused a severe demographic catastrophe. The second reason was the political dominion established under the Spanish colonial system in the Americas, that not only persecuted and enslaved indigenous populations but also systematically killed thousands of them in a practice that many Historians consider a genocide. Moreover, the Spanish colony also eradicated all forms of native cultures, languages, religions, and political systems and supplanted them by Christianity, Spanish language and culture, and colonial rule.
The steep decline of native populations could have been prevented if the encounter between Spaniards and native American would have been less invasive and if the European settlements in the Americas would have been established in a more gradual way. However, it is important to remember that it was a clash between different world views and, many times in History, when this happens often the victorious are those cultures that present more advanced technological and military features.