1) They genuinely think that Gordon Brown has a chance of recovering, and actually want to be part of his Government. Relatedly, they just may not be interested in the leadership.
2) They don't see the point in rocking the boat, Blears style, now, and would rather take over once Brown has lead Labour to oblivion at the next election.
3) In Miliband's case, he may prefer for Johnson to take over before the next election, leading Labour to a less inglourious defeat, before taking over himself.
4) They don't want to get their hands bloody, and are waiting for the backbenches - and that "signupnow" email - to wield the knife against Brown.
5) They are waiting to make their move once the election results are out. As I suggested earlier, the rebels may find it easier to move against Brown once they've got a set of nasty poll results to wave under his nose.
Now, 1) and 2) are laughable, given both the dire position that Brown is in and the rumblings that both men have their leadership campaigns primed. 3) and 5) are certainly possible, although I think 4) is the most likely candidate. Do CoffeeHousers have any other theories?
This leaves two distinct problems for Johnson and Miliband. First, the backbenchers may not succeed, and they could be left stuck in a Brown government (indeed, their own inaction may even make this outcome more likely). And, second, even if Brown does go, they will be rightly asked why they didn't act on principle, as Purnell did; why they colluded with a dying premiership until the final moment. This will weaken their platforms from the outset. Yep, whichever way you look at it, their spinelessness is unedifying - not to mention bad for their own party and the country.