Adam Serwer at Mother Jones thinks he knows why:
Previously, PolitiFact's system for deciding the "Lie of the Year" was through popular vote, which in all honestly seems like a strange way to decide something like this. Nevertheless, while in 2009 and 2010 the lies of the year reflected choices made by readers, as Steve Benen points out, this year PolitFact decided to go with the third-place choice.Read more here.
Explaining this decision, PolitiFact's Bill Adair wrote that "We discussed each of the other finalists and concluded that while clearly false, they failed to be as significant as the Medicare claim." So when Republicans were set to come up with a "Lie of the Year" hat trick, the good people at PolitiFact intervened on an entirely arbitrary basis. How exactly is it a "fact" that the Medicare claim is "more significant" than Senator Jon Kyl's (R-Ariz.) claim that abortion is "well over 90 percent of what Planned Parenthood does," or that "zero jobs" were created by the 2009 stimlus bill? It's not a fact at all. It falls in the realm of "defensible argument," a fact-checking nether region that, as Jonathan Chait points out, at the very least includes the Democrats' claim that privatizing Medicare changes the program so as to effectively "end it."
It seems rather clear that the point here was to avoid another avalanche of conservative criticism that would undermine PolitiFact's credibility as an unbiased source....