Hindus have a right to practice their religion and read their favorite scripture. They can read them in their homes, read them in their places of worship, they free to share their wisdom, they can gift the book to visiting dignitary. This is guaranteed by the constitution of India.
But to fish in troubled waters of multi-ethnic, multi-cultural, multi-religious population is a dangerous proposition.
What would declaring something as a 'national scripture' entail?
It protects it from a 'critical review'. Criticizing the national scripture's short comings will now can be treated as insulting the nation and hence can be treated as anti-national. This can then used to silence any rational thoughts and reform movements in Hinduism, condemning the religion to depth of superstition, psuedo-science and make it the land of curses and spells like in Harry Potter stories.
It is inconceivable that on one hand we send rockets to Mars and Moon, on the other hand believe in 3000-year-old mythological literature. But that would be my belief, a right our constitution gives us.
Just as I don't impose my nonsense on you and force you to accept my belief, no one else has the right to declare their belief as a 'national scripture' and force me to respect it.