It's simple really. Throughout history mankind has shown us a number of things. One of the biggest is people cannot be trusted with power. People who gain power over others trend towards abusing that power. Governments given power trend towards tyranny. I think this 10 minute youtube video really highlights the common forms of governments and how they evolve. The first time I saw it I thought "YES! Why don't more people get this?" and promptly made my daughter watch it.
Topics like this should really be a syllabus for classes in high school. I think how our world governments work is much more useful to people than remembering which general won which battle in some war 200 years ago.