Why was it hardest for the untouchable to live in ancient India?

The untouchables are the lowest caste in Indian society. These people, thrown by fate to a lower social level, are forced to live in poverty and perform the most difficult work. The untouchables could not contact representatives of other castes and had no opportunity to transfer to another caste. The untouchables were assigned separate areas for housing, which they were forbidden to leave. Families are also formed exclusively within the caste.
Their descendants also become untouchables by birthright. The division of labor has made these people outcasts in modern society as well.