There are a variety of reasons why this was the case, but the basic reason is that the Native Americans did not have the weight of a huge population with factories and farms behind them. This gave the US an advantage that Native Americans had no hope of overcoming.
There are other reasons for the Native Americans' defeat. One important one is that they were never able to present a united front to the settlers. Americans were almost always able to use some Native Americans against others. This was due to tribal rivalries and the fact that the Natives did not see themselves as a single group.
However, it was population size and industrial might that really mattered. There were many more Americans than there were Native Americans. More importantly, the Americans belonged to an industrial economy that could produce huge amounts of food and weapons and other supplies. The Native Americans had only such guns as they could obtain from Americans. They had no large farms supplying them. They did not even have a centralized government to collect taxes and pay for an army to defend them.
Because of all these things, the Native Americans had no chance to successfully defend their land against the United States Army.