Well, of course the real reason is because they didn't want to pay Chris Meloni what he wanted so he left. The story on the show about Stabler is that he was trying to help a girl (a rape victim? I think), but then she came into the station house with a gun and starting shooting so he had to kill her and it really messed with his head. Then IA started looking into all the other shootings he's been involved with (a lot!) so he decided to quit instead of dealing with all that.
I wasn't sure if I'd like SVU without Stabler but they seem to be doing well so far. I haven't been watching a lot of the new shows. I like Terra Nova (answer to all objections---things are bad in the future. They're working on a low budget and under bad conditions. They can't afford things like prison security and mandatory sterilization
