These towns are venues for kumbh mela because of mythology. After the samudra manthan (ocean churning) for a pot of amrit (nector of immortality), there was a fight between gods and demons.
Lord Vishnu, disguising himself as an enchantress (Mohini), seized the nectar from the demons. Lord Vishnu passed the nectar to his winged mount, Garuda. The demons finally caught up with Garuda and in the ensuing struggle, a few drops of the precious nectar fell at Allahabad, Nashik, Haridwar and Ujjain.
Another story says that during the fight for the pot of nectar, it was kept on the ground at Allahabad (Prayag), Haridwar, Ujjain and Nashik. And the kumbh mela at these places signifies immortality of soul and prayer to the gods.
Devotees believe that by bathing in the Ganga zone one is freed from their past sins (karma) and thus one becomes eligible for liberation from the cycle of birth and death. This holy event with such tremendous faith can be found in ancient texts of the world.
Famous traveler Huen Tsang of China was the first to mention kumbh mela in his diary. It mentions the celebration of 75 days of Hindu month of Magha (January-February), which witnessed half a million devotee including sadhus, common man, the rich and the kings.