India was ruled by the Britishers(UK) for more than 150+ years. And in the United Kingdom(UK), the financial year starts from 1st April & 1st Jan. And therefore, the same concept may have been applied to India as well by the Britishers and after Independence; the Indian Govt didn’t try to change this Britishers rule.
The second reason may be that the months of November & December are considered as Festival Season due to the presence of many festivals like Christmas, Diwali, etc. The Inventory is very fast moving during this time and it would have been difficult to close the books of accounts.