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Obama is trying to pick the best people for the jobs he has to fill. Hillary, for example, has the respect of leaders around the world, or at least more so than Daschel, Richardson or (especially) Kerry, and, being young and relatively inexperienced, it is probably wise for Obama to pick someone slightly to his right so her counterparts won't think they can get away with something. Also, he needs a strong hand running State, since he is going to be knee-deep in domestic economics for at least the first year of his Presidency.
Whenever I see complaints that all of Obama appointments are Clinton re-treads, what I don't see is a list of alternatives. I think that answers the question as to why he is picking whom he is picking.