It's because the next election is pretty much unwinnable for Labour regardles sof who is in charge. It's better to have Gordon as the sacrificial lamb and then mount a challenge to become Leader of the Opposition once the Conservatives are in power and build your base from there.
If a new leader was to replace Brown now, then an election would be almost unavoidable as they would be the second Prime Minister to hold the job without a public vote (Brown hasn't fought a General Election as PM yet as he took over when Blair stepped down *after* the last election). Plus regardless who it is they'd be fighting a loosing cause. Sure, they might be able to build a few bridges but not enough to keep them residing in #10 Downing Street.
To be blunt, if someone took over now I doubt they'd last too long after loosing the Election. So who, out of the serious and viable contenders, is going to want it now? The best thing any of them can do is consolidate their places and start building alliances so after the next Election they can go for the top job.
Thus Gordon Brown will remain as PM. Not because he's doing a good job and has party and cabinet support but because no one else wants the job right now as it's poisoned. The best thing Brown could do that would send his detractors and plotters within his party off-balance would be to resign now. Then someone has to step forward - and risk getting tarnished by loosing the next election. Of course if they win then they're the golden child and those that held back waiting to step in post-Election loss are screwed.