Why does the poem begin with the Trojan War, not with King Arthur or Sir Gawain?

The poem begins with a lengthy description which establishes the setting firmly in Arthurian Britain. The writer traces the history of Britain from the Trojan War, the founding of Rome by Aeneas, and through to the eventual founding of Britain by the legendary Felix Brutus. Britain is a land of great wonders and strife, but King Arthur has established a court of utmost nobility and chivalry, peopled with the bravest knights and fairest ladies. The poet will now proceed to relate a particularly extraordinary episode from King Arthur's court, which begins at a lavish New Year's celebration in Camelot.