The unbridled truth is that marxist communist and socialist thought dominated the Indian establishment - polity, academia, media - close to a century and still is well entrenched. Their portrayal of history is nothing but dark ages then glorious Islamic rule. And the hostile Islamic neighbour controlling many of the politicians like their puppets using many strategies didn't help either. And the religious conversion lobby were unfettered in creating atrocity literature and demonisation of native culture, mostly with help from enormous foreign aid.

All other reasons people give here are mere technicalities.