I would like to refer you to a wonderful book titled Guns, Germs, and Steel by Jared Diamond. You can look it up on eNotes and find all sorts of information about it--and of course you can read the book itself and also watch an excellent documentary with the same title on DVD. Diamond starts off by saying that he wishes to explain why the Western World got so far ahead of the undeveloped world. He discusses the agricultural revolution which left many people free to engage in manufacturing, how this led to guns and cannons and to steel weapons and steel armor; also how the Europeans carried germs such as smallpox to which other populations had developed no defenses. I suggest that you click on the link below to go to eNotes Study Guide for Guns, Germs, and Steel as a first step.

One possible explanation is simply that the lifestyle of the Native Americans did not require the advancements that the Europeans discovered. The saying, "Necessity is the mother of invention" has become a cliche for a reason; if the way something is being done is working for those completing the task, they have no incentive for seeking another level of technology for the task. Or, if someone does discover an alternative, others may not be particularly interested in changing, if there is not a significant (and obvious) benefit.

The level of interaction was different, so even those improvements in techology that might provide that large benefit were not easily shared and spread, so that the overall technology level might be viewed as inferior. (Consider, though, the Mayas and Incas, and their advanced medical and architectural/structural technology.)

The most commonly accepted answer to this revolves around the fact that the Native American populations were relatively small and could not easily interact with one another.

In Eurasia, there was a huge population with many different civilizations.  Civilizations arose because there were many plants and animals that could be domesticated.  Then the various civilizations interacted with one another and shared the various kinds of technology that they discovered.  No one civilization had to invent everything for itself.

In the Americas, there were few plants and animals that could be domesticated.  There were also fewer people.  This meant fewer civilizations that could interact with one another and share technology.  Each civilization was much more "on its own."