He was coming off a down year at age 28 that in all likelihood will be an anomaly in an otherwise successful career. It's a similar line of thinking as what happened with Josh Beckett's first extension (which came after his first year with the Sox) -- give a player who has stuggled some security, and gamble that you're buying low.
But according to Rob Bradford of WEEI (via MLBTR), that isn't likely to happen. The Sox and Lester have not talked about an extension to this point. This is the last guaranteed year of his contract -- he has a team option in 2014, his age 30 season.
Teams are usually hesitant to give up team options, so I guess it shouldn't be surprising. Still, I would have expected them to have at least talked it over with Lester's representation, seeing if they can lock in his prime at a potential discount rate by doing so early. Let the agents be the ones to say no.
The other way to look at it: Red Sox management had an extremely busy offseason. They spent the majority of the early weeks engaging Toronto on John Farrell. They were as active in free agency as they've been in years. They re-signed Ortiz. They had, apparently, a lot of talks with Cody Ross before he signed with Arizona. They made the Hanrahan trade. Maybe it just comes down to time -- they spent their time on more pressing needs, given that Lester is secured for two years (counting the option) anyway.