The reason why after Capitalism's worst decade in a century, after a pandemic which highlights the need for better social healthcare, as the majority stand still economically in most of the western world the Left is largely politically dead in Europe and elsewhere.
At it's most basic a lot of people in the Left have to stop being horrible twats to their party colleagues, their voting base and target voters.
Forget the opposition or alt right, just stop being poisonous to those 3.
That's not going to happen.
As for capitalism's worst decade, that's due to failed economic models, and the application of dodgy financial ideas. Debt financing is a cancer in western business, just as the debt bubble was for banks. Short term gains for long term downfall. Capitalism is suffering because hard lessons weren't learned, but were merely shifted across industries. The same can be said regarding social policy, where failed systems were simply applied to other concerns, thinking it'll be different this time, even though the same practices/processes were applied.