Someone else will probably have a better answer, but I remember studying in school and later. There were a few different reasons.

1. With empires that stretched all over the northern part of India, their capitals were often too far from the borders. The north eastern and western problems kept them busy.
2. The Vindhya mountain range runs east-west in the middle of the country. In a way, it acted as an effective barrier just as the Himalayas, keeping the south peninsula fairly separate from the northern occupations. Another thing that was mentioned and can actually be seen in the architecture in the south is that in the Deccan plateau (south of the Vindhyas) you'll find a tonne of forts on the higher plateau elevations. Compared to the plains in the north, it would have been difficult to attack from the north because of higher elevation in the south.

This is based on my limited recollection of my university south-east Asian history from India! I hope it helps.

Edited for language clarity.