I think she left because she wanted to start doing movies, and something bigger than a t.v. show. (which didn't work out too well for her.)
And then it ended the season after that because ratings weren't as high after she left. Not as many people like the 4th season. I have to say I love it though, it's different, but it's really funny (after the 1st couple of episodes).
What seasons have you watched?

i saw the first saw the 4th one...(i know its too stupid of me) and then the first one...
i seriously loved the 4th one!!!...it was sooo cute
but after watching the 1st one i could make out why people didnt like the 4th one as marrissa left...