The biggest single factor is probably Macaulay's infamous "Minute on Education" from 1835 which basically created the modern landscape of Indian higher education (and therefore the status of English as the de facto language of power, commerce and governance):

How then stands the case? We have to educate a people who cannot at present be educated by means of their mother-tongue. We must teach them some foreign language. The claims of our own language it is hardly necessary to recapitulate. It stands pre-eminent even among the languages of the West. It abounds with works of imagination not inferior to the noblest which Greece has bequeathed to us, --with models of every species of eloquence, --with historical composition, which, considered merely as narratives, have seldom been surpassed, and which, considered as vehicles of ethical and political instruction, have never been equaled-- with just and lively representations of human life and human nature, --with the most profound speculations on metaphysics, morals, government, jurisprudence, trade, --with full and correct information respecting every experimental science which tends to preserve the health, to increase the comfort, or to expand the intellect of man. Whoever knows that language has ready access to all the vast intellectual wealth which all the wisest nations of the earth have created and hoarded in the course of ninety generations. It may safely be said that the literature now extant in that language is of greater value than all the literature which three hundred years ago was extant in all the languages of the world together. Nor is this all. In India, English is the language spoken by the ruling class. It is spoken by the higher class of natives at the seats of Government. It is likely to become the language of commerce throughout the seas of the East. It is the language of two great European communities which are rising, the one in the south of Africa, the other in Australia, --communities which are every year becoming more important and more closely connected with our Indian empire. Whether we look at the intrinsic value of our literature, or at the particular situation of this country, we shall see the strongest reason to think that, of all foreign tongues, the English tongue is that which would be the most useful to our native subjects.

http://www.columbia.edu/itc/meal...

Macaulay's minute is unpleasant to read with today's sensibilities. It is at least cultural-supremacist but not (praiseworthy in its time) racist. The main argument (that non-European languages could not possibly be upgraded) is easily seen as ridiculously flawed today, thanks to examples like Japanese.

But one cannot deny that a lot of its individual claims are quite defensible. In particular, the most infamous line of all is actually more defensible than it might seem. I am quoting the whole section, though most jingoists usually quote just the bold part:

I have no knowledge of either Sanscrit or Arabic. But I have done what I could to form a correct estimate of their value. I have read translations of the most celebrated Arabic and Sanscrit works. I have conversed, both here and at home, with men distinguished by their proficiency in the Eastern tongues. I am quite ready to take the oriental learning at the valuation of the orientalists themselves. I have never found one among them who could deny that a single shelf of a good European library was worth the whole native literature of India and Arabia. The intrinsic superiority of the Western literature is indeed fully admitted by those members of the committee who support the oriental plan of education.

Exaggeration? Yes. Wild, indefensible exaggeration? In 1835, based on what he could have known then, no.

Why talk of Macaulay? His influence dictated the development of Indian educational institutions. Particularly higher education. As other parts of the minute reveal, his strategy not only promoted education in English, it actively discouraged the further development of the two main Indian scholarly languages (Sanskrit and Urdu) which were not yet quite dead back then (Sanskrit is basically dead today; Urdu does not survive as a scholarly language. It survives as a literary language to some extent). Other languages (Tamil etc.) were also carefully neglected, though they had fewer institutions stewarding them to begin with.

Though arguably Indian languages could have developed and caught up with European languages if they had received proper patronage, that, unfortunately, did not happen, and Macaulay's vision came to be realized. The result is people like me, unapologetically anglicized in their thoughts and words and (accented and idiosyncratic) speech. A strident nationalist movement popular about 10 years ago used to call people like me "Macaulayites" (Nehru was the first).

That leaves the question of Hindi. Credible candidate to replace English?

No.

Hindi was born a politicized football of an artificially created Frankenstein language that squeaked through the Constutional Assembly as the "National Language" by one vote. It never had a chance in hell of becoming a true national language, and it never did. Thanks to Macaulay, none of the real languages had any realistic chance either. The parts for the Hindi-Frankenstin came from a bunch of real North Indian languages like bhojpuri, maithili, khari boli etc. etc., de-urdufied and sanskritized that suffered even more as a result of development resources being shifted to "Hindi."

Now, all Indian languages are basically stuck in the 19th century. They have no institutional or high-culture political foundations to serve as languages for modernity, to talk about things like the Internet, space exploration, DNA etc. A brave rearguard whose social identity is anchored around specific regional languages continues to fight and carry on a limited literary tradition in various languages, but all consequential engagement of modernity happens in English.

Attempting to pick and promote an Indian language (even assuming all the political problems and chauvinism could be dealt with) would be about as hard as attempting to upgrade the ENIAC operating system to compete with Linux/MacOS/Windows.

So basically, game over. English it is. At least as long as India remains a single country. If it breaks up into a dozen smaller countries (could very well happen in the next 100 years), then interesting revival of regional languages might happen.