Because the British government was taxing them heavily.
Nation is its citizens and Jefferson stresses basically that that is why the people must participate in the government. The government is of the people for the people which means that the government is made up from the citizens of the United States. It is those same citizens that, by their right to vote, choose who will do their will in the government. Precisely because the people choose who will be in the government is the reason why they must be more active as it will decide what policies were going to be pushed.
