Blunt, despite his stated reasons, dropped his reelection bid because he could read the writing on the wall. Democrat Jay Nixon would have destroyed him in the general election.
Everyone, even Blunt, knew that. It would not have been pretty. Though his endorsement could help with fundraising and possibly push Hulshof or Steelman to the Republican nomination, associating too closely with such an unpopular governor would hurt that nominee's chances against Nixon in November.
Governor Blunt, please stay out of this. For the sake of the party, it is time for you to head back to Springfield and just fade away.
(Link to source blog at stltoday.com.)