As earlier stated by fellow member basically in Southern India people are reluctant to converse/interact in National language .
Are you claiming that Hindi is the national language, it is the biggest misconception for majority of the people, India does have 22 official languages and there is no national language. English is used in every state and you can adopt your regional language for legal proceedings.
On point, about the shudh Hindi. However, people don't speak shudh hindi anymore either.
And its not about reluctance, have you ever tried to learn the southern languages? say for Tamil for instance? I have, unsuccessfully. The vernacularity, word structure, bending of the tongue everything is different, they simply unable to speak Hindi - let alone any northern languages.
People from Tamil Nadu does speak Hindi and Urdu depending on their religion, but majority of them does not speak Hindi simply because there is no use of the Hindi language during his entire life if he is staying in Tamil Nadu or abroad India, if a non Hindi speaker is living in a Hindi dominant place he will definitely learn the language in a few years time.