Because it was single symbol Ghandhi found that could be unite the whole nation i,e rich and poor as it is a constitunent food iteam used/needed by every sector of society and they wholy take part in Salt March.
Gandhiji wanted to revolt against the British by marching till Dandi in Gujarat and making salt. … Salt was something consume by everyone rich and poor and it was one of the essential items of food. The tax ans salt and the government monopoly over its production.