Because of the strike they wrapped up season two early. Season two was originally supposed to include two volumes (Generations and Exodus) but after they re-shot stuff for the finale it seems they've ditched the Exodus volume altogether since the "current" volume is now Villians.
Ha, now that you're asking I'm wondering if season two wasn't just put on hold, but I'm 85% sure that is the case.
"And where do you live, Simon?"
"I live in the weak and the wounded... Doc."
- Session 9