This is a great question that almost nobody asks.

It's can be difficult to pinpoint, because the subjugation of women has many active/passive components that can be different/similar across cultures and history. Anthropology often chronicles these, and the power dynamics around them - but overarching theories are rare.

To me, the most convincing theory uses a Materialist framework, and has to do with social reproduction. Materialists argue that economic factors (human labour, the products of it, and the means of its production) are the centre around which social organisation develops - and that hose who control these, control society.

Now, the means of production for most things is human labour (and, for the last 250 years, machinery/technology/capital). But the means of reproduction for society itself - i.e. the social reproduction, the production of more humans - are women. So while it would make sense that women ought to have power over this (and in society), it also makes sense that men would seek to subvert and control that (and by extension them).

How could this happen and how could people let it continue? Well, the reason nobody asks the question and few theorise on this, is a good explanation of why.

Hegemonic ideas (ideas which come do dominate in a society) are very all-encompassing, invisible, and often unconscious. People can easily learn (even without being explicitly told) that this group is 'different', and different can come to mean lesser. People are also very attached to the ideas they acquire.

Women exerience ourselves as normal human beings of course, and it's human to baulk at being subjugated and be outraged at being mistreated. But it's also human to adapt to circumstances, and exercise agency within them, even if less than ideal.

The alternative - you'd also be surprised and shocked at how effectively, violently, yet implicitly women who resist the behavioural/social roles imposed on them (e.g. conceding in conflict) have their behaviour isolated and corrected by horizontal power dynamics (i.e. the people around them). Humans start to pick up social dynamics aged 3 months. By the time we reach adolescence, we have been socialised into these roles and ideas so much so that it's near impossible to extract them all - and attempts to claim/exercise more power are usually swiftly shut down.