Why did third watch end?

"Third Watch" ended after its sixth season primarily due to declining ratings and changes in network programming strategies. The show's creator, John Wells, felt it was a natural conclusion to the characters' arcs and stories. Additionally, the network, NBC, shifted its focus to other programming, leading to the decision to cancel the series in 2005. The series finale provided closure to many of the character storylines, wrapping up the show's themes of heroism and community.