theory

A combination of simply lacking the technology to advance production and the rise of the division of labor. Marx has a quote from The German Ideology that is immensely useful:

We shall, of course, not take the trouble to enlighten our wise philosophers by explaining to them that the “liberation” of man is not advanced a single step by reducing philosophy, theology, substance and all the trash to “self-consciousness” and by liberating man from the domination of these phrases, which have never held him in thrall. Nor will we explain to them that it is only possible to achieve real liberation in the real world and by employing real means, that slavery cannot be abolished without the steam-engine and the mule and spinning-jenny, serfdom cannot be abolished without improved agriculture, and that, in general, people cannot be liberated as long as they are unable to obtain food and drink, housing and clothing in adequate quality and quantity. “Liberation” is an historical and not a mental act, and it is brought about by historical conditions, the development of industry, commerce, agriculture, the conditions of intercourse...[There is here a gap in the manuscript]

This is the basis of historical materialism. Early slave societies rose and then transformed into feudalism through successive improvements in production technology, development, and transformations in the division of labor. When early communalist societies began agriculture, this gave some groups more time than others, which gradually transformed into social leverage and an actual class, state, and more. This is also why communism abolishes the division of labor as such, and why many socialist countries try to focus on polytechnical education and development of the productive forces.