It is the mother who creates new life. The creator of the world would naturally be the mother of the world. The mother cares for the child, giving it nourishment and love.
The experience of union with everything around you is associated with the feminine and thus the goddess. (It's also associated with marriage.) You can find it in all the versions of mother Earth, mother Nature and Gaia.
The mother becomes the center of the family, explaining the needs for caring of the children. That makes them matriarchs. When the family grows with a second generation, the role of the grandmother will shift from caring for their children to caring for the future of the larger family. The grand grandmothers would quite naturally be seen as representing the motherhood for the whole tribe.
Ecofeminism combines the sacred feminine with environmentalism and politics.
From Horizon Zero Dawn meeting with High Matriarch Teersa:
We teach, offer counsel, give blessings. When necessary, we judge. Our only concern is the welfare of the tribe.
Among our number this year we count three High Matriarchs and some thirty lesser Matriarchs.
Matriarchs are grandmothers with two generations of living progeny. But a High Matriarch is a great-grandmother, with three or more generations to her name.
Thus do we speak for generations, lending out opinions weight – the High Matriarchs most of all.