Michael Rosenbaum talks about why he decided to walk away from Smallville

On his podcast Inside of You, Micahel Rosenbaum elucidated the reasons behind his decision to exit Smallville in season seven. 

Even though it has its problems, Smallville is still one of the most influential Superman- and, by extension, DC comics- adaptations created in the 21st century. When Smallville first debuted, the series presented younger fans with their first look at the Man of Steel's formative years in Kansas. Consequently, Smallville is still regarded as the most definitive version of Superman for certain fans. 

But what's a hero without a villain? During the first seven seasons of Smallville, Michael Rosenbaum delivered an amazing performance as the show's Lex Luthor. However, Rosenbaum unexpectedly left the series despite copious praise for his portrayal. Although, with the benefit of hindsight (the declining quality of the series and the horrific Nxivm cult associated with the series), it seems like Rosenbaum was the smartest man in the room- which is kind of on brand for Lex Luthor.