Because many on the left were already like that back then.

They were like that in the 1930s. A couple of years before WW2 the English Labour Party voted to disband the RAF because its existence was upsetting Hitler and making him feel insecure.

There was certainly strands of looney leftism going back into the Victorian era. People saying you shouldn't criticise slavery or human sacrifice in other countries because you can't think your own culture is superior.

Frankly, I think it probably stated back at the French Revolution.

Yes, that gang were comedians. John Cleese, from that very gang, had a scene cut from his magnum opus Fawlty Towers, but I don't remember why.

I think the issue was racial.