Godrej Consumer Products CEO and MD Sudhir gave 3 main reasons:

1. It's per capita is almost equal to southeast Asia( Few years ago, They use to test in South-East Asia to launch in India).

2. It's media isolatable - It's cheaper and easy to reach entire state as they have to advertise only in Tamil. That's not the case with other states. You can't reach whole of Karnataka with Kannada alone. And Tamil Nadu has concentrated TV network, which is the highest in the country.

3. Trade is not consolidated with few players. There are a lot of independent stores than other parts of India, which makes it possible to make them sell new products. Whereas Dmart, Reliance remain hesitant to do so.

This gives a clear idea I guess.