Simple. He knew that he couldn't win.
Some might think that when the Dems took the Senate this past Tuesday, staying in that body became more enticing for Feingold. Maybe that just gave Feingold the excuse he needed to bow to the inevitable regarding his presidential prospects.
The announcement of his non-candidacy is about as shocking as the one made by Mississippi Republican governor Haley Barbour a few weeks ago. (Who even knew that Barbour, the former Republican National Committee chair, was considering running for President?)
I guess now Democrats and Republicans will have to content themselves with getting involved in the campaigns, respectively, of Tom Vilsack and Duncan Hunter.