Corbyn is a hardcore anticapitalist whose aim is a socialist society (soviet block style). He is in favour of Brexit, too, for this reason. Same goes for McDonnell, a Marxist.

That's very far left and is opposed by most people including in the Labour Party. Basically, those guys were the Communist Party and unsurprisingly people did prefer to keep the Tories...

It was the same in the 80s when Labour was essentially unelectable. Blair saw that the centre left was politically the best bet, and pragmatically that social policies needed the private economy to produce wealth.