It was a combination of things, mostly entirely predictable.
But the thing I didn't see coming at all, and that annoyed me the most (still does actually), was the strength of opposition within the Labour Party. I naively thought that after a MP from the left of the party had been properly elected leader, most MPs would quieten down and help push the rather attractive policy agenda forward. But right from the start when Mr C struggled to put a shadow cabinet together because the talented candidates all refused, through the chicken coup to the hopeless second Brexit Referendum policy, via Labour politicians vowing to "do something every day to undermine Jeremy Corbyn", he never had a chance. The 2017 referendum was a remarkably good performance, all things considered.
Oh well, any Labour government will be better than any Tory one, even if it doesn't move us much closer to a socialist nirvana!