Why wasn’t Supernatural renewed yet?

Right TV has an article questioning why the CW has yet to pick up Supernatural for a fifth season.

Note: Supernatural was picked by the CW (following the WB and UPN merge) on May 18, 2006. The series was renewed for a third season on May 15, 2007 (2 days prior to the CW upfronts), and was picked for a fourth season on March 3, 2008 (following the writers' strike).