I just cant believe the festival of Diwali have been shown such extend of indifference by you sir!! How can take to your heart that it is not a festival of Kerala too!!Hindu mythology is something for everyone who follows the religion with no partiality between south or north.Let me try to explain...
Diwali is celebrated not only in India but also in Sri Lanka, Myanmar, Thailand, Malaysia, Singapore, Indonesia, Australia, Netherlands, Fiji, Africa, Canada, Mauritius, New Zealand, UK and even the US. Of course the presence of Hindus is one unavoidable reason. It is celebration of hearts depicting the victory of good over the evil, a celebration of relations and love.
The celebration mainly focuses on different myths where the dates coincide with each other. The Birth of Goddess Mahalakshmi symbolized by the myrth and the lights that we kindle the day to symbolize the prosperity of a home which is why the purchase of new utensils and gold is believed to be an inevitable part of the celebrations.The festival also celebrates the destruction of the demon Narakasura by Lord Krishna which is the victory of good over evil.The other myth is of Lord Rama returning with his wife Sita and brother Lakshmana after destroying the demon King Ravana who have stole away his wife. The return as we know is from Sri Lanka to Ayuodhya. The role of Kerala in the festival is at this point when Rama , Sita, and Lakshmana are welcomed and shown the way at night with every houses lit with small Diyas contributing to the beauty of the night. This is the reason why the festival is celebrated in each state a different day. It is simple logic as the couple and Laxmana travel from South to North they reach each place a different day. The last reason that contribute to the festival is the return of Pandavas from exile and hence the reason of showing the way with lighted lamps as well as the return of relations back home. The festival hence symbolically depicts the prosperity of the house, binding of relations with love and care which ensures our protection from evil.
Each religion has its own other reasons for the festival.. Jains celebrate the festival to celebrate the birth of Mahavira and consider it as a New Year. Buddhist celebrate Diwali as the day Emporer Ashoka converted to Buddhism. For Sikhs the day is a memory of their spiritual leader Hargobind's release from prison and also honor the construction of their Holy city of Amritsar. The epic of Ramayana is also the basis of the celebratio of the festival in places like Thailand and Singapore where the epic is modified according to their culture.