Because the Skull Kid has cast a spell over the giant moon to come falling down to Earth in 3 days. Link can use this time loop to his advantage to get 4 of the boss remains in 4 dungeons, do many side quests, collect all the items, get all of the pieces of heart. And do side quests involving completing Bombers notebook. The time loop will end if Link can call the 4 giants to stop the moon and defeat the final boss.
