That short a distance out from the election they need a leader who has instant name recognition among the electorate - that candidate doesn't really exist in 2014's Labour Party, but Alan Johnson is probably as close as you get. David Miliband would be better, but by November 2014 he's on Tracy Island.
But Labour isn't faced with a hopeless election like the Conservatives were when they elected Michael Howard, so Johnson isn't being put up to lose an election, so ambitious candidates can't afford to unite behind a single candidate - if they decide to put (rolls my Labour MP dice) Alistair Darling in charge there's a chance that he actually ends up in charge of the largest party, if not holding a full majority, come May 2015, and Burnham, Cooper, Alexander etc. have to hold their fire until he can be convinced to step down.
So you have a situation where Labour need a coronation, but there are too many candidates who can't afford a coronation, so you end up with a clown car leadership election which drags on into early 2015, and then a very surprised-looking Ben Bradshaw has a couple of months to answer the question, "Who?" or hand the Conservatives the election.