My latest question which asks comparing India and Europe which have multiple languages. One commenter downvoted as to how I can even compare Europe with India! Reminded of untouchability in India! look at the audacity of racism of beta exchange!
I think that the question you're trying to ask is "why is India a single country, rather than several countries, given its ethnic and linguistic diversity?"
This is a good question, but:
It's much more about history than politics. I think history.stackexchange.com would be a far better place for it.
Your comparison to the EU is strange and muddles the question, making it unclear what you're really asking. I suggest you leave this out, especially because:
The tone of your question comes of accusatory. It looks like you're trying to accuse the European colonizers of poisoning India's future, or something. Trying to make a point in a question almost never makes the question better, even if it's a good point, because once you start making points in questions it stops being a question and becomes a statement ending with a question mark.