Why does Purna Kumbh only come once every 12 years

Kumbh mela, an event that Hindus look forward to each year, and every 12 years, is one that is observed and celebrated with great pomp and show. It is a fair of spirituality, purification, and new beginnings for many.The term ‘Kumbh’ translates to pot, and ‘Mela’ simply means fair. It is said that after the Samudra manthan, when Lord Vishnu was carrying away the pot of Amrit from the asuras, a few drops, 4 to be exact, fell on the ground in 4 different locations. And these locations then became the spots in which Kumbh takes place.