because she didn't want to do another season on the show. She also wanted to branch out and jump to doing movies, obliviously didn't work out for her since she is going to be on a new series starting this fall on the cw.
I also heard that one of the reasons was that Mischa, who played Marrisa, was always turning up late to work and that this annoyed the rest of the cast but I don't believe it. I think she just wanted to try something new and work on new and different sort of projects. :)