Why did a war break-out in the Heavenly Kingdom? (Revelation 12)

Because “free will” is extant in the Spiritual Realm as it is in the “Physical Realm.” Satan’s rebellion is causation for the Creation Narrative…all that we’re experiencing in the spiritual war raging-on around us is acting as a bulwark against future rebellion in the New Jerusalem where “free will” will be extant by necessity.

This is why war between Jesus and Satan was first declared in Genesis 3:15 and the battle strategy, the Gospel, was first proclaimed in Genesis 3:21. Jesus entered Time specifically to defeat the works of Satan (death-Hell-the grave) at Golgotha. The entirety of Scripture surrounds and supports the victory of Elohim over the Evil One; this, for the benefit of Elohim’s divine relationship with the angelic creation that remained faithful in the Great Rebellion.