I'd imagine it's because they played their hand last summer and it backfired. If they'd been strategic and realised that Corbyn would've fell on his own sword then they'd have been in a much stronger position.
The majority in the party are just desperate to get rid of the Tories. It doesn't matter what faction the candidate comes from, they just need to come up with a coherent plan to get rid of the Tories and people will vote for them.
And what does Corbyn "falling on his sword" mean? He didn't resign after the failure to gain any concessions to prevent a "bargain basement"(his words) post-Brexit economy, or after whipping his MPs regardless of whether they were in Remain constituencies or not to pass the unamended Article 50 bill. He didn't resign after the Copeland embarassment (which seems to have been largely over concerns about his personal views on nuclear energy). He hasn't stepped down despite apocolyptic polling numbers. Add to that Traingate, and the Trident confusion last week and yet he seems as comfortable in his position as Leader as ever.
So he seemingly has no self-awareness and/or willingness to take responsibility for his failings, and consider resignation and the PLP are effectively barred from criticising him so like I said before, what are the Corbyn-supporting Labour supporting members going to do about it if, as you claim, they are simply desperate to get rid of the Tories?
The answer to that question, unfortunately, is that I think it absolutely matters what faction of the party a leadership candidate comes from. Seemingly if you're in favour of actually explaining how you'd fund your spending; if you don't feel our nuclear weapons are one of the more pressing issues at the moment; if you feel air strikes and other such military action should at least be an option, you're a dirty Blairite who should be dismissed. I actually have a lot of sympathy for the views of Corbyn supporters on Trident and I didn't support Trump's air strike in Syria (and not just because it was done by Trump) but I also don't believe it is the right time to kick up a fuss over Trident, nor would I rule out voting for a leadership candidate that did condone Trump's strike.
Existing Corbyn supporters either don't have the ability to compromise or don't understand why hijacking the party and forcing it away from its long-standing position on issues i.e. on the EU, or on Trident alienates fellow Labour members/supporters. If you want to take those stances on issues, preferably set up your own party or if you are going to force your views on others, don't play the victim when they turn away from you.
