Because the numbers make it tricky, and the Lib Dems and Conservative rebels are being intransigent, saying they will not support a Corbyn lead GNU. They are even insisting he stand aside and let someone else - Conservative veteran Ken Clarke or Labour's Harriet Harman - take over.
This is all down to politics, I think. The Lib Dems and pro-Remain Conservatives just don't want to be seen to be enabling or to give him even the whiff of credibility as a PM.
The moment Corbyn gets to stand up in the Commons as PM, or speak to the nation from outside Downing Street, and the sky does not immediately fall, he gains stature and his enemies within and without the Labour Party lose their best argument - "It's impossible to imagine him as PM."
They seem willing to risk No Deal rather than let Corbyn be PM. They may fold - or he may fold, which is why they are saying they won't countenance backing him. The irony being they are basing their argument on the same logic the ERG are using when they say we can not take No Deal off the table - because it would take pressure off the other side.
Whereas - if Corbyn does say he will stand aside for another candidate - The issue them becomes pro-Brexit Labour MPs. They would support Corbyn as PM even if it meant nixing Brexit. But they won't swallow their principles for Ken Clarke and I suspect they wouldn't do it for Harriet Harman either.
(Both these figures are being touted as benevolent 'national treasures' who no-one sensible could refuse to support; Clarke spent decades enabling Conservative governments; Harman has been a hate figure on the right for decades as well, though she did do some good things - 'Harman's Law' - also described as 'socialism in sentence' was a splendid idea, and repealing it was one of the great wrongs of the Cameron administration.)
Which means that if the Lib Dems and Conservative Remains won't compromise to support a Corbyn lead GNU, forming such an animal under the leadership of someone else becomes much harder.
Corbyn and Labour have already compromised massively on Brexit.
Yet it never seems to be acknowledged; instead they are accused of "sitting on the fence" by extremists on the Remain side. Labour are told to be pragmatic and sensible, but the same expectation id not demanded of Lib Dems and Conservative rebels who say they will refuse to support a Corbyn lead GNU.
If we do crash out, it will be cause of their intransigence. The numbers are there, but they are refusing to support the best placed candidate. And it is for political reasons, as outlined about.