It is not. Because even nation states are a modern concept, borrowed from Western countries. If that is acceptable, then so is multilingual states. Otherwise, why do we not go back to the Hindu and Buddhist kingdoms as it existed back then? Within this modern nation state, we have to accommodate many many interests. Language is destiny. To get Hindi promoted over rest of the languages would be criminal injustice to many other language speaking citizens by not only giving them an unfair advantage in govt jobs and entrance exams but also be responsible for the slow suicide of all other cultures and languages.
There is entrenched cultural value with languages too. For the most part Hindi that is spoken is not easily differentiated from Urdu and as such carries a large component of Islamic values through it. From disdain and contempt for sadhus and sants to temples, and means of worship in temples, there is a large difference in the outlook of those states which were under Islamic rule (Hindi speaking states) and the states which never endured Islamic rule for any significant length of time to have altered their native culture.
They follow vote bank because they know there is a constituency which aligns with the view they are promoting.