Gangajal is considered very sacred in Hindu religion. That's why it is advised to keep it in the house, but then why it is prohibited to keep the idol of Maa Ganga in the house. Mother Ganga is considered holy, worshipable and mother in Hindu religion. That's why from Ganga bath to keeping Ganga water in the house has been said to be important and beneficial.
❀ According to the scriptures, Mother Ganga is a river and the nature of a river is to flow, so if an idol of Mother Ganga is kept in the house, it shows the flow. That is, nothing is stable in the house, everything is unstable and in flux.
❀ Even in the scriptures, it is considered prohibited to keep a divine idol bigger than 6 inches in the house. Whereas keeping a small idol is like obstructing the source or size of Maa Ganga. In such a situation, there will be obstacles in the happiness-prosperity, peace, opulence, wealth, progress etc. of the house.
❀ Due to its lava, it is also believed that Maa Ganga is a cursed river. Mother Ganga has been cursed to be dirty. Although the water of the Ganges is considered sacred. In fact, Ganges water is a form of water, hence it is sacred.
❀ But the idol of Maa Ganga is in the form of a goddess, so it is inauspicious to keep it in the house. It is believed that Mother Ganga washes away the sins of all people. Keeping his idol in the house means that everyone in your house has sins.
❀ In such a situation, you may also have to bear the consequences of the sins of others.
That's why it is auspicious to keep Gangajal in the house and keeping the idol of Ganga is prohibited. That's why they don't keep the idol of Maa Ganga in the house.