One, he's the choice of the Rethuglican clique whose smear campaign torpedoed Ambassador Rice's candidacy -- Arizona's Sen. Grumpypants, and South Carolina's Sen. Huckleberry Closetcase. They'd love to crow about how they forced President Obama to kneel before them, especially over a politically-manufactured "scandal" that the Ambassador wasn't involved in. In particular, Grumpypants is still in a white rage about his defeat in 2008, and this will only embolden his continuing efforts to sabotage the President.
Two, if Kerry would become Secretary of State, it would open up his Senate seat for a special election, which Rethuglicans believe former Sen. Scott "Centerfold" Brown would win. The name of Victoria Kennedy, Ted Kennedy's widow, has been mentioned as an interim appointee until the special election this summer. Special interests (banking and pharmaceuticals in particular) would throw big money behind Centerfold in an effort to reduce the Democrats' margin in the Senate, putting any Dem contender at a disadvantage. Why the White House would want to hand such an opening to the Rethugs would be a mystery, but stranger things have happened.
UPDATE: How about this for a nominee: Deputy Secretary of State William Burns, our top career Foreign Service officer, who has served in Republican and Democratic administrations, and was Assistant Secretary for Near Eastern Affairs and Ambassador to Russia, among other assignments. Qualified and bipartisan.