Because it is not based on a "good Utopian man of the future" as anarchists and Marxists believe. The government is not a solution for economic problems but is needed to set universal rules the courts can decide upon. There is no recipe for a perfect contained small government, but our Constitution was a good start that could be improved step by step.
The problem is with people themselves. Ron Paul sadly did not study Ayn Rand enough and settled for Rothbard instead. Natural rights come not from god or nature, but from reason. Unless majority of the population (or majority of intellectuals) adapt RATIONAL MORALITY, no clever scheme for a free society will work as planned for a long time. I am sad to report to you that religious folks do not hold rational morality. It is a mixed bag.