This is something I've raised in this thread before, the left don't know how to win.
In the last election the Lib Dems and Labour stole votes from each other whilst the Brexit party stood down in seats where it was going to be closely contested. The left don't care about winning, they care about purity and 'being right'. Its morally laudable to say that a LD and Labour candidate might disagree on some things, but in the main they agree on one major thing: the disagree with the Tories. However rather than being pragmatic, they compete over the minor issues rather than focussing on the bigger picture and working together to win a left-wing coalition.
In the 2019 GE the popular vote was almost an even 50/50 split between the left and right parties, but the Conservatives won a thumping majority. In my own constituency the Tory (the only right wing candidate) got 49.5% whilst Labour, LD and the greens combined for 50.5%. People voting LD and the greens might have thought they were doing the right thing, but in reality their votes directly elected the Conservative MP. I cannot fathom the mindset of someone who vote LD or greens in a tightly contested seat.
For the left to win, tactical voting must become the norm.