why do you specifically say Allah

There is more than one God in the earthly civilizations, why do you specifically say Allah?

There is no true deity in all the religions of the earth except Allah.

The point of difference with the rest of the religions, is that they took along with Allah other gods like Jesus and the Holy Spirit in Christianity, Vishnu, Shiva, Brahma in Hinduism, etc.

All the religions believe in Allah the one and consider him to be the creator of the existence.
But they associate other gods with him in worship. Please consider the verse (interpretation of the meaning):

Even the idolaters did not worship the idols for their own selves, rather they acknowledged that Allah is the Creator, and they took them as intermediaries.

Shahristani says about the ancient Arab idols
As for the idols, the Arabs did not worship them for themselves, they did not consider them more than pieces of stones"
Dur Durant decides that the origin of the pagan Hinduism is ultimately due to the belief in Allah the only true God.

He says: "These thousands of gods are the same as the Christian churches of sanctifying the thousands of saints. the Indian may not ever think that these gods, which are of infinite number, have the supreme sovereignty.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: It is not correct to assume that the worshipers of idols believed that the idols created the world or that they cause the rain to descend or that they cause the plants to grow or create the animal or other than that of the divine attributes. Their aim was the same of those who worship the graves". 

The idols and the human gods, are but the means of infidelity, for which Allah has sent down no authority.