Via Ben Smith at Politico. And, like Ben, I imagine this has at least something to do with Hillary Clinton's interest in the position. This makes it harder to accuse Obama of slighting women if he offers the job to a man. And insofar as it's older women, predominantly, who make up Clinton's base, they may be more likely to accept a non-Clinton alternative if it has the imprimatur of a Kennedy.
For the record, I'd expected to see some elder party statesman like Tom Daschle or George Mitchell in this role. And I have no idea whatsoever whether Kennedy is particularly qualified for the job. But the search committee also includes James Johnson, who's apparently done this sort of thing before, and campaign legal adviser Eric Holder.