وسبب وجوب الغسل الحيض والنفاس والجنابة
The source lists menstruation, postnatal bleeding and major ritual impurity as causes relevant to obligatory ghusl.
Although both wudu and ghusl concern purification, they do not follow from precisely the same underlying circumstances. Ordinary minor ritual impurity is not, merely by being impurity, a cause of obligatory ghusl.
A person with minor impurity requires wudu for prayer; major impurity requires ghusl. Menstruation and postnatal bleeding carry their additional specific rules concerning cessation and purification.