Why do you think the Native Americans were unable to defend their land against the U.S. Army?

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.