The word allah is unique you cannot have more than one  Allah only one Allah for example the word Allah's cannot be used. So when the word Allah is used you know it refers to the one and only creator. On the other hand the word God is not unique it can be said as god's for example the Greeks God's and so when the word god is used it is unclear as to which God is being mentioned. Is it the Hindu God or the Greek God or the Muslim God?