When they announced that the character was leaving, they reported the development as a creative decision.
She brought intelligence, a black-belt in aikido, and baggage from the murder of her twin sister along with her to the Crescent City.
It seems things have spiraled from bad to worse in the four years since.
The infiltration by Russo also opened the door for a new story arc on the show.
Apparently, the agent's storyline involved dating Homeland Security agent John Russo, who she discovered was working as an anti-government mole.
Exactly why did the show after just two seasons? I heard from his mom not too long ago over emails.