The quick answer is: because too many already have.
If Corbyn was likely to lose, I would almost certainly have put him first, because there is a very real danger that the party would overreact to the disappointing election result by ditching Ed Miliband's timid move to the left and signing up to most of what the Conservative government currently stands for, but with a little less brutality towards the poor and immigrants. An attractive proposition indeed. But if the polls are anything to go by (and I remain a bit unsure that they are, given the difficulties involved in sampling in this kind of election), it looks like he'll probably win. And I'm pretty sure that is not a good outcome, at least in the short term.
But there are many many good reasons for voting for Corbyn, as long as he doesn't actually win. For a start, I find myself agreeing with just about everything he says. Sure, he's been too close to extremist elements in the Middle East, but so has the British government. Yes, he is talking about quantitative easing without showing a clear grasp of how and when it should be used. And he has a beard. Nobody ever wins elections with a beard. Or at least, nobody since Disraeli. But basically, on the key questions of our time - austerity, inequality, the role of the state in the economy - he is actually the closest thing to the zeitgeist we have. Innovative and smart thinking about the economy - Piketty, Mazzucato, Summers, Haldane - is much closer to the kinds of things Corbyn is saying than to any other major political figure in the UK. That doesn't necessarily win you an election of course, but it does suggest that you are asking the right kinds of questions, and could even have some answers.
There is no point in my rehearsing the arguments against Corbyn, because we are being bombarded with them from all quarters. Most of the time, these arguments are boneheaded and facile: he can't win an election (polls show none of the others can either, for now)! He's old-fashioned! He has shared platforms with people who hate Israel! However, there is no denying that if a basically honest, decent and intelligent guy like Ed Miliband can be destroyed by the press, there is little doubt what they will do to Corbyn. Most of the likely propaganda will be a pack of lies and gross distortions, but it will be effective enough to stop him winning over the kind of essentially conservative middle income English voters that Labour has to win over if they are to get into government again.
But I very strongly believe that whoever does win the leadership election, or take over when Corbyn finds he cannot actually control the parliamentary party, needs to take on board a good part of what Corbyn is saying. Not only because I think he's right, but because strategically it's good politics.
First, if we think Conservative economic policy is wrong, because it is depressing living standards unnecessarily and penalising the poor, then you have to make that case, otherwise it is hard to see how you can do anything different when you are in power. In particular, Labour needs to be ready for when the results of the Tory economic experiment come in. I believe that the experiment will prove to be a failure, and that it would be much better for Labour to have been opposing it all along. Of course, you have to do this in a smart way, and not open yourself up to the charge that you just want to print money (even though that is an option under certain conditions). But you have to be ready for the economy to tank (it may even happen as soon as this autumn). After all, if the Tories are right and the economy does improve sustainably and grow living standards, Labour will lose anyway.
Second, the stock of older property-owning voters that have kept the Conservatives in power won't be around forever. In the next 5-10 years the generation of people shut out of the property market and penalised by austerity will start realising that things aren't going to get any better for them, and will demand change. The older voters will die. Immigrants will have children, claim citizenship, and start to vote. Demographics currently favour the Tories, but they won't forever. Labour needs to have a progressive agenda that can appeal to the changing electorate. I don't think they can make many inroads amongst the current over 65s, but they can mobilise the younger electorate, and at some point that may well produce a winning coalition.
Finally, of course it is true that to make change you need to win power, and that involves focusing on the possible rather than the desirable. I lived through the 18 years of Tory government before 1997, and can remember the desperate need for it to end. But I think we are currently operating with an out-dated and one-dimensional view of what the possible is. First, in the 1980s and 1990s free markets were an exciting new (if rather old) idea whose time had come. By now, the limitations of blind market worship are coming clear, and the distributional effects of Britain's particular brand of neoliberalism are damaging the living standards of the majority of people. The Third Way made sense in the 1990s, it makes little sense now. Second, the electorate has splintered, and winning is not just about occupying a central position and expecting SNP, Green, UKIP voters and abstainees to suddenly flock to your door. We no longer have a two-party system, so more than ever winning is about building a coalition. It is not at all clear to me how signing up to Tory economic policy and hostility to immigrants, the poor and the young helps you do this.
So, these are the reasons I'm not voting for Jeremy Corbyn, but wish there was someone more credible out there ready to take on board much of what he is saying. I'm holding out for Yvette Cooper, with Andy Burnham in second. And as for Liz Kendall, well, I think she's on the wrong side of history.