They are terrified that people with an actual progressive agenda could spread in the party. If you’re familiar with the Labour Party during Corbyn there was open contempt for their own progressive membership from the long standing party insiders. Getting rid of those members was described as ‘shaking off fleas’.

But I don’t think the main reason he is being frozen out is his progressive policy platform, but that he doesn’t parrot the right pablum about how Israel has the right to defend itself as the answer to any atrocity. Corbyn was also tarred as an antisemite, but that was more of a pretext rather than the actual reason. In this case I think the lack of bootlicking for Israel is the core of the issue.