Very good question. She was my favorite character and I was sad to see her go. Shortly after she left DS9 I began seeing her on the show Becker. Bad move!
After six seasons on DS9, Farrell was presented with the chance to join the Paramount sitcom Becker. Instead of renewing her original contract for DS9 for one more season, she left and joined the cast of Becker in 1998 where she stayed for four years playing the character of Reggie Costas. There were many rumors as to why Farrell left DS9; some believed she wanted more money, there were differences with producers, or she had arguments with fellow cast members. Her stand-in and photo double from DS9, Cathy DeBuono, also worked in the same capacity for Farrell on Becker.
After leaving Deep Space Nine, Farrell stated that, if asked, she would love to return to Star Trek to play Jadzia as a clone, Jadzia from another universe or a hologram. She also stated she grew greatly from playing Dax and that she wouldn't change a second of her time with Star Trek.
Farrell's character, Jadzia, was not seen as a part of the flashback sequences toward the end of "What You Leave Behind". This was due to the circumstance that Farrell did not give the producers her permission to use pictures of her in those scenes. Although a scene was written for Jadzia in the final episode, budget costs meant they couldn't afford Farrell and the scene was never filmed.