It was out of fear for the Avatar. You see in our world there is one human born each generation who has the power to manipulate all four elements, and serves as a balance to the world. The only being capable of putting a severe dent in the plans of conquest Fire Lord Sozin had for the world. The cycle goes Fire Air Water Earth and constantly repeats. The previous Avatar Roku had died, and the next reincarnation would be within the Air Nomads.

As destiny would have it, the Avatar escaped capture, and went dormant for a hundred years, until he awoke, and began his journey.