Our complicated and immensely flawed "hero" taking charge of the defense of King's Landing isn't merely to fulfill the stock fantasy archetype but is meant as a culmination of his emotional journey throughout the season.

Source: https://www.slashfilm.com/1238505/the-battle-of-the-blackwater-from-game-of-thrones-season-2-is-the-best-action-scene-ever/

The TV show kicked off with the seeds being planted for the Targaryen civil war, so most of the series focused on King's Landing.

Source: https://screenrant.com/game-thrones-seven-kingdoms-name-reason-nine/