Because Corbyn had the best shot of the bunch presented due to the momentum and energy behind him.
Again you are looking at this like you're some sort of alien who doesn't understand human beings at all.
Jeremy Corbyn WAS an electable Labour leader. Over the course of the campaign he became an unelectable Labour leader when the PLP undercut him, the manifesto went too far left on some very easy to point to economic issues that the electorate wouldn't like, and because him and his team were heavily linked to the IRA and we couldn't get the message through the noise to shift it. Then due to these issues he became unelectable, with the terrorist one being the biggest issue IMO but debateably so.
See how that works? That if you don't have a zealoted belief in something that you can change your mind on an issue without it being a religious war when new ideas or information is presented?
Your timelines are a bit peculiar, I'm not quite sure at what point Corbyn became unelectable in your eyes. The vote of no confidence by the PLP triggered the second leadership challenge, in which you voted for Corbyn and of course preceded the election campaign, which produced the manifesto, a manifesto credited with giving Corbyn a real boost, so tell me, what was it, precisely, about this socialist manifesto, drafted by an avowed socialist, that took you by surprise?
Corbyn's IRA links have been knocking around for some time, at which moment over the last two years did it tip over in to being an issue?
Oh, I'm interested to know, why is it that to believe in something is zealotry and yet to believe in something, then believe in something else and then nothing at all is pragmatic?
I understand politics, as in life, it is essential to compromise, the Labour party knows that, but there is a line where compromise becomes capitulation, its name is New Labour.