There are, I imagine, two main reasons for this. First, it's all too soon: Labour won't want to engage in full internecine combat while there's still the chance of a deal with the Lib Dems. And, second, they will want to create the impression that – contrary to Gordon Brown's ascension to power in 2007 – the next Labour leader has been chosen in an open and clean contest. This, they no doubt hope, will dilute the calls for an immediate election should that leader also become the second "unelected" Prime Minister in a row.
Which is why the Tories should do all they can to expose the cogs and gears already operating behing the nascent leadership contest. Whether it's the Unite money lubricating Ed Balls's bid, or the Dark Forces mustering behind David Miliband, we're about to see a stunning display of an unapologetically old politics. And – if it comes to it – it sure won't be a proper substitute for a mandate to govern.