India - Southern states vs Northern states

Why are Indian southern states more developed than most northern states?

1. Size. It is not north vs. south, but small vs. big. Except for Maharashtra (which has Mumbai to power it), all the other top 10 states in percapita incomes are smaller ones (Goa, Himachal, Haryana, Delhi, Sikkim), while the biggest ones (Bihar, MP, UP, Rajasthan) rank the worst. Even in the south, Kerala (smallest) scores way higher than AP (largest) in most development indicators. Size really does matter, when it comes to development. Smaller states can provide better governance and can focus more on development. Southern states are smaller than those in the Hindi heartland. 

2. Impact of partition & invasions. Most of the upheavals that India faced in the past few centuries was borne by the north: Afghan invasions, Han/Mongol invasion, partition. South was untouched, comparatively. This let us protect our institutions better and focus more on growth as soon as India got its freedom.

3. Access to sea. Historically, nations & states with access to oceans have always done well, given better trade, fishing, moderate weather, more rains, transportation options (boat) & more exposure to outside world. Think of US coast vs. midwest, Japan vs. Mongolia, etc. Southern states & Gujarat have plentiful access to the ocean. The Hindi belt is disadvantaged here by being landlocked (Also see: Landlocked country).

4. Lack of major benevolent kingdoms in the recent-past: After the fall of Gupta empire more than a 1000 years ago, north India didn't have a major home-grown empire. Mughals did good in patches, but were mostly undone by barbarians such as Aurangzeb and were sometimes considered aliens. However, in the past few centuries, Maharashtra had the Marathas, South had Vijayanagara empire, etc that provided a little better base to our development.

5. No major cities: Hemanshu Desai pointed in the comment about cities. That is a fair point. Haryana has Delhi in proximity. MH has Mumbai, TN has Chennai... Kerala is highly urbanized, although it doesn't have major metros. The problem with central/north India is that there are no modern cities in proximity to support 600 million people of the area.This requires a major rework on developing cities like Patna, Lucknow, Bhopal and Ranchi.

But, it was not always like that. Bihar and Punjab had among the oldest Universities (Nalanda and Taxila, respectively) and for 2000 years, Patna was the major center for India and led India in most aspects. Along with Kanchipuram, Varanasi was the major center of learning. Some of the greatest Indian kings - Ashoka and Samudragupta came from this belt. Hope, they get inspired by the glorious past.