Why should education be made a fundamental right?

If we want to live in a democracy with educated and participating citizens who each have a chance to succeed in life, then the way to accomplish that is to make education a fundamental right. In fact, in my opinion, that is the only way to achieve this goal.
A democracy is only as good as the people it represents, since the electorate makes choices about who will represent them. An uneducated and uninformed citizenry makes poor choices. This can result in a failure of democracy. It also explains why it is difficult to export democracy to countries and cultures where education is not a fundamental right. Good democracy is a skill that must be learned. 
A democracy should want a happy and self-actualized citizenry so the democracy will be productive and stable. People who are more educated tend to be happier and more productive because they have the wherewithal to solve problems and think critically, making better choices and finding pleasure in productive and challenging work. They also tend to be more self-actualized, as their education provided a good set of tools with which to become that way.
Finally, education is usually the best way up the socioeconomic ladder. It can break the cycle of generations of poverty, allowing young people to begin a new cycle of, if not prosperity, than at least not abject poverty. We all want our children to succeed more than we have, and education is a way to accomplish this. Without the right to education, this cannot happen very often.  
John Dewey, one of the greatest educational philosophers in modern history, wrote an entire book entitled Democracy and Education a hundred years ago. I have included the link to it below. You might want to read some of his ideas, too, about why education must be a fundamental right.