Sorry, let me rephrase that question.
Why do you think is the cause for these social conventions and gender roles?
The universal standard force governing human life and social activity is custom. We are creatures of habit, and naturally - like all things in nature - we attempt to live via a path of least resistance. This is pretty much the root-level behaviourism that governs all social structures, group psychology, and culture (including the perpetuation of religious belief). It becomes custom; 'second nature', whatever you want to call it. Gender rolls probably in early primitive/tribal societies had an organisational role: the basic dualism of hunter-gatherers and home-makers. They've persisted since then because, well, our evolutionary biology and psychology hasn't really changed all that much since those times.