Because of the terrible situation regarding human rights in China. Here in the Netherlands, there is a lot of discussion about even a boycott by the sporters (OK, so we won’t really boycott it, but at least the discussion’s there), and, somewhat more realistically, a boycott of the ceremony.
Then again, I often hear on the news that this discussion is hardly present in other countries, with the exception perhaps of Sarkozy.
(I do wonder why the IOC decided to hold the games in China in the first place, as I’ve heard a representative say they had expected all this controversy…)