Rafael Behr in the Guardian:

The Fixed-term Parliaments Act gives the Commons 14 days to organise a replacement when an incumbent government is defeated in a no-confidence vote. Who else is going to lead that administration if not the leader of Her Majesty's opposition? In constitutional terms he is the obvious candidate; probably the only candidate.

But in the minds of scores of MPs he is not. His past equivocations over Europe are not the reason, or at least not the only reason. Pro-European Tory rebels, Liberal Democrats, the rag-tag platoon of independents and semi-autonomous tribes of Labour MPs have spent months fretting about ways to thwart a hard Brexit, apparently ready to pull every procedural lever and contemplate all manner of unorthodox coalitions. Not much has been excluded from those considerations, except for a tacit prohibition on any route that makes a prime minister of the current Labour leader. Their horror of Corbyn is equal to - or greater than - their horror of Brexit. That has been so well understood by the participants in the discussion that few have felt much need to articulate it. Corbyn's letter now obliges them to spell it out.

(...)

There is something disingenuous about the discussions among MPs about a "government of national unity"(GNU) to avert a no-deal Brexit. It is predicated on concepts of nation and unity that don't include those who are desperate to leave the EU. Those who voted leave are broadly satisfied with the government they currently have. It is, in truth, a euphemism for a model of technocratic, centre-facing liberal administration defined as much by a rejection of Corbynism as by revulsion at the Trumpian nationalist character that Brexit has acquired.

(...)

The Labour leader knows this and he is calling the whole GNU bluff. If a government falls, the opposition leader is the next in line to have a go and, if that can't be arranged, there is an election. That is how it works. There might be many reasons why MPs do not want an opposition leader to take charge - that is their constitutional right, too - reasons of tactical political advantage and reasons of conscience. But MPs have not all been candid about what those reasons are; why it is that so many find Corbyn as toxic as Brexit. Their problem is that there aren't a lot of other options. And the laws of political motion are working against them.