It was explained. The land originally existed as one called Okkarra. It was then split with the one we know being called Krakoa and the other half, Arakko . It remained dormant in the Pacific until nuclear testing awakened it and it then went on to do the events of GSX and was shot into space. Pieces of it fell to the Earth and grew into seperate beings, also called Krakoa. They would be used in several stories with the most noticeable one being in the WATXM run.
While this all happened, the main island was taken by the Stranger and experimented on until it was freed by Quasar and made its way back to the Pacific. Xavier traveled to it and offered and through Doug, convinced it to be the ground for the new nation