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.

He won't have the support and political capital to pass most of his agenda but the opposition fears the groundswell of support for his policies and want to delegitimize it before it grows.

The establishment is worried that his success will spark a wave of actual progressive candidates instead of do-nothings who talk a weak opposition game and then cave under the slightest pressure.

He is wants to introduce state ran grocery stores with lower prices than chain stores, freeze rents for some people, and give people free childcare. So like basic human empathy stuff. Its still treating the symptoms of capitalism but its treating them as directly and effectively as possible. His platform isn't that radical, but any concessions to the working class that doesn't make someone a buck will always be faced with hard opposition by those who benefit from the status quo.