It's not actually unitary, it's devolutionary. Not sure how to cite my constitutional law professors here but a major point of the course was that any countries start federally and centralize, while others start unitarily and decentralize. When the decentralization is asymmetrical - that is, Scotland has more power and competence than NIreland (and even votes on affairs related only to England) - then it's devolutionary. There really aren't many devolutionary states, although the french treatment of new Caledonia may apply.

Australia, Canada and the US were organized as a series of discreet colonies, and formed nations after the fact. When the nations were formed the states / provinces had significant political and cultural value to their inhabitants and a federal structure accommodated that.

Unlike the case for Australia, Canada and the US, where British emigrants could quickly overpower and ultimately outnumber the aboriginals, it was expedient for the British simply to co-opt the existing Indian princely states and to establish regional governments overarching them and settlements and colonies in between them ... so India would have faced the same problem (dissolution of long-standing political units) at independence, and did not do that at least at first. (India went on to make the princes abdicate and did some significant consolidation, but the political core of federalism remained intact.)