That is because of the slave mentality that has been created in India whereby people are ready to learn a foreign language so keenly and yet consider it a personal insult to use a common Sanskrit-origin language for pan-India communication. In fact, southern and eastern Indians would find it the easiest to learn Hindi (north Indians don't care much about state accents, unlike English where their incomplete words are to be pronounced in the correct manner).
The slave mentality stems not from common people but from the ruling Indian National Congress that made English like a livelihood of India. Knowing the international language is good, but here in India, people are obsessed with it. Obsession is wrong and ruins personal respect.
There are Japanese, Thai, Russians, French, German, Hong Kong-ers, Chinese, Taiwanese, Japanese, Koreans etc who are far far well-off and richer compared to India. They don't use a single word of English and still have managed to have some of the most prosperous economies on the planet.
If these countries are homogenous, that is not an excuse. In India, all governments give 100% freedom and encouragement to state and regional languages. All media channels have big businesses in regional languages such as serials and movies. All the state governments are welcome to retain their languages. Hence those who say that Hindi is being imposed on them are bullcrap liars. They simply want to gain cheap political mileage out of maligning an easy-to-learn pan-Indian language of Indian origin.
My language is Dnzongkha but still I speak and write fluent Hindi. Majority Sikkimese, Arunachalis, Ladakhis and I'd say even to a warm extent Manipuris are very open to the language and know to speak Hindi very well. It helps us connect to a Marathi or a Punjabi or a Gujarati. That's the main advantage. Feeling a sense of connectivity without having to use a foreign-colonial language.