Well, Allison Mack being a punk and not wanting to do one more freaking season on the show that made her known has a lot to do with it. At the same time, they probably want to focus more on the "real" mythos to get us to Superman without aftermaket characters dragging them down. All love in the world to Chloe and AM but, really, she should've just done one more damn season and Chloe isn't a "real" character in the Superman mythos and we need to get this shit done. So, really, her being gone will probably let that happen. Somewhat.