As was suggested by another FReeper, I think Matheson is stepping down now to preserve his viability for 2016 in order to run against Sen. Mike Lee (since the RINO establishment will do all it can to take him down, and if they can’t primary him, they’ll back Matheson).
I think it’s more likely he’d run for Governor, he could conceivably win that. I’m not worried about Lee, should I be?