Thousands of years back, people bursted crackers everyday! They were addicted. They loved it! But then the pollution rose, the temperatures rose, the carbon dioxide and sulphur, and global warming came. The government and people were scared of global warming. So they decided a single day, a single day like Environment Day. But on this day they were allowed to burst crackers! But then they won’t be allowed to burst crackers the rest of 364 days of the year. They called this day, DIWALI. So ever since on Diwali people burst crackers to fulfil their year long urge to burst crackers and to celebrate their 364 days of saving environment from harmful fire crackers!
Just a theory by me. Does not exist.