The contrast between individualism and communitarianism is highly metaphysical, but also is a matter of historical timeline. From Ancient Greece right up to the time of the Middle Ages, the metaphysical systems of Plato and Aristotle dominated the philosophical scene, and since both of them wrote political treatises, their influence was felt in that area, as well.
Of course, looking back you see the atomism of Democritus, the radical materialist/nominalist Sophists like Gorgias and Protagoras, the nominalism of Ockham, the deviation of Duns Scotus, among many other thinkers who pulled away from the traditional metaphysics of the time. But suffice it to say, the universalism of abstract objects was a central postulate in communitarian systems. For a variety of different reasons, the community was seen as the ontological ultimate, a natural institution as real as any other organism, capable of giving moral commands which bind individuals, whether or not those commands are based on fairness or fact. For Socrates, it was social contract theory. For Plato, the state mirrored that of the form of the state, and the tripartite division of the soul, with reason signifying the rule of the state and its status above all others. For Aristotle, the whole is greater than the parts, either individually, or in sum, since the whole exists logically prior to any of its individual members. Near all of these systems—especially Aristotle’s and Plato’s—depend on universals.
One can easily see the problems that would emerge from a denial of their metaphysics. And this denial can be felt in every area of philosophy, since each of Aristotle’s and Plato’s positions were deductions from metaphysical postulates.
Thus, when someone like Hobbes comes about, he eschews the idea of the soul as a myth left over from the fanciful Greeks. Despite the Leviathan being a work of political philosophy, a sizable portion is dedicated to writings on the church and on psychology. The more famous portions quoted endlessly in anthologies are actually relatively small. Since Hobbes is an ardent materialist, he has no time for the world of forms, the souls, talk about ‘wholes’ existing apart from parts, etc. For Hobbes, it’s clear; only individuals and physical things exist. The state cannot be natural or an organism, since nature is utterly devoid of purpose. Thus, any organization that exists is merely convention, the result of contractual arrangements between individuals, which again are conceived of as atoms.
Now, clearly Hobbes does not want to commit himself to a historically extravagant hypothesis. He really has no way of discerning whether this was the case for all existing states, so he confines his theories to a moral and rational justification of the state. He allows for many different varieties of state-origin theories.
Looking back at the trend started by Hobbes, MacIntyre points out the underlying metaphysical battle going on through Hobbes and the Enlightenment. Hobbes had an intense dislike for Aristotle and the Scholastics, and so was happy to discard their systems, but this has led to the modern malaise, the breaking down of a conception of community that has a higher goal in mind than the selfish, individual conceptions of the good, which last about as long as the length of a television show. Walzer and Sandel join MacIntyre in this, although it’s MacIntyre (as well as Taylor) who place extra emphasis on metaphysics.
For them, theories of human nature are important, as well as is talk about rights and obligations; they would emphasize the notion of positive freedom over negative freedom—the liberty of the ancients over the liberty of the moderns. Thus, obligations are not something you just decide to take on through an exercise of autonomous will. That makes little sense to the communitarian who holds that obligations exist as a part of one’s identity, and it is not possible to accept them, since they just exist as brutal facts, so that acceptance would just be redundant.
In more recent times, the most obvious example would be the criticism of Rawls’ veil of ignorance. According to Sandel and others, the idea that we can shed our religion, culture, gender, race, etc., is to shed us of what makes us—us—at all. When analyzing the actions of actors, one cannot create an artificial entity in an artificial thought experiment to illustrate justice. This makes little to no sense at all.