Remember, there was a significant third party showing in 2008, which probably took more Franken votes. Plus, Coleman was an incumbent. Neither of those are the case this year.
As well as the fact that Franken is now running as an incumbent Senator and not as a former SNL cast member and controversial talk radio hack.
Yeah, I think Franken lost his footing during a negative race and hadn't proved himself in 2008. People in Minnesota are fine voting for a liberal, but they recoil at someone who seems like a pushy, loudmouth. Franken just kept his head down for 6 years and built a progressive record.