Why is there only one God in all religions and in Hindu religion there are so many?


God likes variety. God did not just make egg plant or just potatoes. Just imagine having only potatoes all your life. 

He made different types of interesting vegetables. When God has made so many different types of vegetables, different types of flowers for you, would the same God like to be in one uniform? He will be wearing different types of costumes. 

And so he will manifest in different names and different forms. He has the right to do that. 

He who likes variety and has created so many varieties of forms also is available in different names and forms. That is why we say one God but with different names and different forms. That is why they say, see ‘One’ in everyone, and see everyone in that ‘One’. This is what is ‘Vishwaroop Darsan’, where Krishna showed Arjuna that everything was in him. ‘Rudra, Shiva, Ganapati, it is me who is present in all forms.’ 
So do not think Gods as separate from one another. God is only one, but there are different Devatas.
Just like how one cell becomes the human body. A drop of semen and one egg joined together forms one cell. From this one cell the human body is formed, but in this there are 33 different aspects. One aspect becomes the eyes, one aspect becomes the ears, and one becomes the throat and tongue. The textures are so different. 
Your nails are different from your eyes and eyes are different from your ears. These are Devatas. That is what the ancient people have said; in our body Devatas are present. In each organ there is a Devata which means divine quality. All these divine qualities, all these Devatas are one God’s manifestation. These are 33 chromosomes derived from the one cell. That is why our ancestors have spoke about 33 crore Devatas.