In 2012, the partisan composition of the Senate class up for re-election will be 21D-10R. In 2014, 20D-13R. Even if the electoral climate in these years is neutral, we stand to lose seats. We finally have a serious shot to gain ground in 2016, when it's 10D-24R. So I can imagine a bunch of Democratic Senators sitting around back in January and thinking, "Why should we pass filibuster reform now? If Republicans take the Senate in 2012 or 2014, it'll just be used against us. And anyway we can't actually pass anything with a big GOP majority in the House. So yeah, filibuster reform is a good thing that should happen, but let's wait on it."
If this is right, it was a good thing for Merkley and Udall to at least talk to their Democratic colleagues about filibuster reform after the 2009-2010 Congress while sentiments in favor of it were high, even as the tactical case was at its ebb. Maybe in 2015 or 2017 when the tactical case is huge and sentiment is weaker, they'll be able to remind people how they used to feel in late 2010.