For decades, the Indian independence movement and its Congress Party papered over those differences through corruption and patronage, but the contrivance ultimately failed because it was incapable of producing the sound policies and sense of national identity required for a modern nation-state.
Not only Spain, but virtually every European country had a long and often bloody history before a solid national identity was achieved. And just as national and cultural identities helped pull apart the Soviet Union and its universal ideology, so also is resurgent nationalism tearing at the seams of the EU.
I used Spain in the example because it was recent; I guess Yugoslavia would have been even better (because Spain at least didn’t have so many different religions to contend with: they primarily had Catholics and ex-Catholics). When you realize we are almost 250 years old as a country, and India hasn’t yet reached 75, you understand they need time.
The EU had issues from the start because not only was it a hodgepodge of various languages and religions, but also cultures; the Germans in the north simply cannot share a “nation” with the Greeks in the south. They have such divergent views on government, work, taxes, etc. that they simply won’t meld...ever.