Why many people in India especially poor not sending their children to school despite free education in govt. schools?

The answer to your question is already covered in our study material. Follow the given link and scroll down to the topic 'Reason for poor education status in India' to view the same.

https://www.meritnation.com/cbse/class11-commerce/studymaterial/economics/indian-economic-development/unit-iiihuman-capital-formation-in-india/442_2487_6388#slide1_Education as a Source of Human Development, Future Prospects