The O.C. was cancelled for a few reasons:
1.Josh Schwartz, the producer & creator of the show believed it was a good time for the show to end.
2. The show's ratings went down more & more by the season and had "only" 4 million viewers by its last season.
3. The hit T.V. show "Grey's Anatomy" appeared at the exact same day and time, which was why a lot of people didn't watch.
I think that when they decided Marissa's death, the show didnt't have any more meat, as for most of us, whether we liked her or not, we must admit she was really a basic character.