Because the Constitution is essentially the most liberal document upon which any government was built, because it states that the government draws its power from the citizenry, empowering people over the government. This type of individual liberty is most present in the modern Libertarian party philosophy. True liberalism is about empowering individuals, but today's liberals are just left-wing statists who use the authority of government to suppress anything of which they disapprove.
In fact, the modern Democrat party are incrementally turning into socialist-fascists.