I think there's a number of factors at play in the collapse into Labour:
The weakness of the extra-parliamentary left.
Labour putting forward what is ostensibly the most Left-wing platform in most of our living memories.
Few people want far left policies so Labour put forward nationalization and other far left policies. Add the leader is a pointless wanker that makes Johnson look like a better choice (That's pretty hard to do), and you have what is very possibly going to be an absolute smashing at the polls. It's too late for this election and prolly Brexit, but the whole party needs a big shakeup before the next one.
I'm going to get hammered by the far left for this post, but that's because they're too fucking stupid to realise a mildly left but electable Labour party that will take care of the NHS and the rest of the welfare state is far better than years of tory shit.
I await being told I'm a wanker for wanting an Electable Labour party that'll stop the tories fucking up the country.