explain why some children in India do not get access to education, even though the Indian government have made education compulsory and free for all children 6-14 years of age.

Explanation:

Search out Greg Mortenson on the internet. With the help of a ghost writer, he wrote Three Cups of Tea. It has moved a great many people including me. It is the story of a mountain climber who wandered aimlessly in the a poor region of Pakistan (but the same comments apply to India). He collapsed in a village where, had he landed anywhere else, he likely would have been left to die.

Eventually after dealing with all sorts of problems (including the Taliban), he comes to realize that the only way out of poverty the people of this village endured  (and many others like it) was education. From that moment on, he dedicated himself to raising money to build schools in areas like the one that found him. His firm belief was that Education was the only answer.

I think the answer to your question is isolation. Some areas of India are just too isolated and too poor to do anything about education. A whole much better description is in 3 cups of tea and if you are serious about your question, you will find it and read it.