Because, especially in recent decades, the vast majority of Americans tend to value individual freedom over collective action; though we may differ as to the actual definition of 'freedom'.
However, many modern day Republicans, especially ones like Huckabee, are increasingly communitarian/populist. Also, the Democrats used to have a very strong populist wing (especially in the South).
I really don't think most Americans value individual freedom more than collective action. Has libertarianism improved its numbers compared to Populism? Yes, but I still think that Populism is much more popular than libertarianism in America.