Answer:
Because a government that has been established for a long time and is still in place, is likely to be a stable, reliable government that acts in the best interests of the people.
The government of the United States, while not perfect, is a remarkable example. The United States is the oldest modern democracy of the world, and the U.S. Constitution is also the oldest still in place. This is because the U.S. government was founded on solid principles that are still valid, and most political authorities in the U.S. agree that the type of goverment should remain the same in most cases.