First of all there are more job opportunities in south of india, even for blue collar workers.
And you will be surprised at the adaptability of blue collar workers. They pick up and learn the relevant language very quickly. Lot of cooks who work in Bangalore are Oriyas(from state of Odhisha).. and they pick up Kannada and other local south indian languages very fast as it helps them find more customers and more ways to earn money. Same for lot of labourers from any other north indian states or Security Guards(lot of IT companies in South Indias big cities), and they are in huge demand.
As a white collar worker, well you need to know only English for employability. But does not hurt to learn a few local words like I picked up few words of Kannada as it helped with grocery shopping and other stuff.
I have seen the same thing in developed countries e.g. in Spain there are lot of Pakistanis running businesses and vegetable shops and working in stores etc. and there Spanish level was A-Class. I went there as an expatriate from an IT company and had company sponsored Spanish courses, but still did not reach same level of Spanish because, well, it was not a mandatory requirement for the job.
So, bottomline is, Economic opportunities determine how fast people adapt and learn a language. It was Persian and Arabic at some moment of time in history, Sanskrit etc. in India and in current global world, that language is English.
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