Why Lord Vishnu took Kurma avatar

Lord Vishnu had pledged that he would appear every time someone threatens the existence of humanity. He had vowed that he would take an avatar on earth to restore Dharma by uprooting Adharma.

Lord Vishnu incarnation of Kurma Avatar starts with curse of Sage Durvasa to Lord Indra that all Devas power will be drained.At the same time ,Asuras started torturing Devas.Due to the lack of powers ,they could not win over the Asuras.So they decide seek help from Brahma.Bur Lord Brahma suggested to seek help from Lord Vishu.Now all the Devas went to Vaikunta and met Vishnu.Atfer reaching all Devas ask help from Lord Vishnu,so He suggests to SagarManthan(churning of Milky ocean) to get amrit and regain the powers.He suggested the Devas to ask Asuras for help in lifting the mountain, in exchange to offer a portion of the nectar of immortality that would ensue from the churning. Both the parties agreed to the pact and started to churn the ocean using the serpent Vasuki as the rope.

Now to do Sagar Manthan,Mount Mandara and Vasuki(snake God) as rope where used.But churning was not possible as the mountain was kept on a flat surface.Then where the Lord Vishnu takes the avatar of Kurma(half human and half tortoise).The Mount Mandara was kept on the back of Kurma to churn the mountain.So,this  is why Lord Vishnu took Kurma avatar.