Because it was the most powerful symbol of resistance against British in India
It all started with salt taxes in the 1830s. A most common commodity in the kitchen, salt was indispensable to Indian homes.
Despite that, or because of it, the Britishers introduced steep taxes on salt and strict laws on its manufacturing. These laws inspired Mahatma Gandhi's Salt Satyagraha in 1930, which culminated into Civil Disobedience Movement.
The historic Dandi March took place from 12th March to 6th April 1930, in which hundreds of Indians marched 385 kms from Sabarmati to Dandi, with Gandhiiji.
Initially, Salt Satyagraha was not taken seriously by the British or even the Congress Working Commitee. But Gandhiji was clear that an iteam of daily use will resonate more with masses for greater political rights.
As it spread like wildfire, as the British realized how the masses are awakening to the idea of freedom, and came down harshly on the movement. The Salt Satygraha was an inspiration to world leaders like Martin Luther King and Nelson Mandela