Original: This tradition originates from the Tokugawa period when the population of Edo (Tokyo), being considerably wealthier than the rural poor, were more susceptible to beri beri due to their high consumption of white rice, which is low in thiamine.
Simplified: At that time, the population of Edo (Tokyo) was considerably wealthier than the rural poor, were more susceptible to beri beri because of  their high consumption of white rice which is low in thiamine.