She was a judge for three seasons (2009-2011), and her departure was part of a larger shakeup of the show. After those three seasons, the show replaced the judges with a rotating panel of celebrity guests for a season, before bringing in Jennifer Lopez, Steven Tyler, and Randy Jackson.
It's important to note that DioGuardi's exit wasn't necessarily negative. She was a popular judge, but the show was looking for a change to revitalize the format and bring in a new audience. While it wasn't a firing, it was a decision by the producers to move in a different direction.