Why did European exploration not result in the improvement of the native people?
European exploration did not help the people that the Europeans came into contact with because it was not meant to help them. The Europeans did not really intend to "civilize" the people they met. They did intend to Christianize them (which would have seemed like "improvement" to the Europeans) but they were not trying to bring modern civilization to the people whose lands they "found."
The goal of European exploration during the Age of Exploration was to make the Europeans richer and more powerful. Because of that, the Europeans (for example) used the Indians of Mexico as practically slave labor in their silver mines rather than trying to educate the Indians so that they could become "modern."
Later on, Europeans and Americans at least talked about the "white man's burden" and making their colonies modern. But at the time of the first exploration, there was no intent to do this. The first explorations were really all about benefitting the Europeans, not about helping the people they "found."