Like, why did it swing toward consolidation of power rather than deeper forms of egalitarianism? Probably crisis, scarcity, natural disaster, war. We have to think about these systems globally and locally. The conditions didn’t exist in one place to develop into castes and classes, but they did in the next area over. Scarcity of resources or opportunities to accumulate material or social resources drive expansionist, imperialist development. What makes the process tick, according to Marxism, is a dialectic relationship between opposing forces.

Spinoza has a line in Ethics which I think is interesting:

After men persuaded themselves, that everything which is created is created for their sake, they were bound to consider as the chief quality in everything that which is most useful to themselves, and to account those things the best of all which have the most beneficial effect on mankind. Further, they were bound to form abstract notions for the explanation of the nature of things, such as goodness, badness, order, confusion, warmth, cold, beauty, deformity, and so on; and from the belief that they are free agents arose the further notions of praise and blame, sin and merit.

If this has some merit, which I think it does but there’s def problems with Spinoza though you gotta love his efforts to construct human nature from first principles, then the “cardinal sin” of class society is one part of society, which is a part of nature, begins to consider itself above and beyond nature. But what drives this IMO is a slow process of material need which leads to accumulation of social capital, that negates the old egalitarian material basis and replaces it with an informal caste that negates the old formal egalitarian basis for human society.

The truth is you are probably just gonna have to work to find specific answers to your question, in my experience these questions exist as a sort of discomfort that drives us to seek answers. Recognition of some objective factor conflicts with something subjective within you that drives you to seek an answer outside of you. This inside/outside, negating old ideas with new ones is the motor of revolutionary criticism. Something like this process happens to people when society goes through changes. This happened to those people who formed those old egalitarian societies when they followed external pressures to change their own societies, just as it happens to us. This inside/outside struggle is the driver of human and social change. All social questions resolve themselves in the practical activity of the people who make up the society. In order to really understand, we have to get rid of abstraction and seek evidence.

But since these class societies arose at different times and in different ways, and much of the old ancient records didn’t survive, then we can look at more recent examples of revolution to understand social change. Detailed analysis of the French revolution, the Russian revolution, the American civil war, the Haitian revolution, WW1, WW2, the Chinese revolution, and the times leading up to them, can provide some insight.

Black Reconstruction by WEB Dubois is one of the best such analyses. He really gets into the weeds of the classes, the opposing forces, the material and social negations, that led to a drastic and necessary material change to the means of production in society. Another good example is Gramsci’s study of the Risorgimento and Passive Revolution. But just by the way you ask the question, seems to me like you are on a personal journey that won’t feel satisfied until you’ve basically transformed yourself and the way you think about the world. At least, that’s how it worked in my experience and from what I’ve seen in the experiences of others