Why was Katie Cassidy replaced on Supernatural?

Why was Katie Cassidy replaced as Ruby on the show Supernatural? I loved Katie. Not only absolutely gorgeous, but as an actor portraying Ruby's character. Then during season four they replaced her? Can someone tell me why? I thought it'd say so on Wikipedia, but i scanned through and didn't find... show more Why was Katie Cassidy replaced as Ruby on the show Supernatural?

I loved Katie. Not only absolutely gorgeous, but as an actor portraying Ruby's character. Then during season four they replaced her? Can someone tell me why? I thought it'd say so on Wikipedia, but i scanned through and didn't find anything. I don't like the new actor playing Ruby.
Update: Why was she fired though.
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