I think it is partly because of the strong association of closeness with the language one speaks. Most leaders of a national stature come from the Hindi-speaking zone, hence a feeling of closeness. The southern states, especially Tamil Nadu, have succeeded in taking advantage of the language divide, labeling it as the Aryan Dravidian problem, which is obviously false. But unfortunately, most persons in India get easily swayed by emotions, passions, and propaganda.
The other factor is that the feeling of Nationalism is strengthened when there are serious threats of invasion. The southern states have not experienced that for a long time, hence the focus has been on regional rather than national issues, which some seem distant.
It is also a realism that there has been a large number of conversions in the southern states, as well as quite a large exodus of the educated class from the southern states to various countries outside. Most converts feel their allegiance more to the Western and the Arab world than to India as part of the mass mentality. Hindus, on the other hand, identify much more strongly to the land where they feel are all their physical, cultural, and spiritual roots. Though here also caste divides create a further division of votes among the religious hindus as well.
Finally, it is to the credit of the leftists that they are experts in the art of propaganda and narratives, which has a large influence on the masses.
There are surely exceptions, and quite a few of them. But democracy is all about generalisations and narratives to sway the masses.
Affectionately,
Alok Da
