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I think there is an ideological difference that many Tory MPs would be reluctant to bridge, around the role of the state in the daily lives of the electorate, that means they'd never cross to Labour.
Electorally, most of the 'one nation' Tory MPs are in south eastern constituencies where their main challengers are the LibDems, so joining them gives them a better chance of remaining as an MP after the next election. It's clear now, I think, that with Badenoch and Jenrick as the two remaining candidates, that the Tories are going to go after Reform voters, rather than trying to stop the LibDems from taking more seats off them.
Why the libdems when Labour is actually in power? Starmer's already shown he'll take some of the more extreme Tories if it suits his purposes (whatever they may be).