I am a big fan of John Kerry and would have loved to see him elected President. He has a presence that suggests authority. He has a good record in the Senate. He may not be the best choice in the whole wide world, but of the names who were considered, and as a following act to Hilary Clinton, he is a great choice.
Making Senator Kerry secretary of state opens up his seat for another election. It's a lot of money the party would have to spend defeating Scott Brown again, or another challenger.
That's assuming that they will actually win it again. In the real world, there's no guarantee that'll happen. And with how much effort it takes to get anything at all passed in the senate, every last seat is crucial.
John Kerry is capable of being Secretary of State, but I do not think that he is Obama's best choice. That would be the current Secretary of State, Hilary Clinton. Kerry just is not as qualified for the job as she is. Mostly due to Clinton being overqualified for the position.
There are a lot of things to think about when choosing a secretary of state. John Kerry should be looked at, but he may or may not be the "best" choice. I don't know exactly what makes up the perfect Secretary of State, and I'm assuming that those in charge will know better than I. However, I think that there is a lot of politics going into this choice, and that alone will make it so the best choice is not appointed.