Ah, perhaps you are less familiar with the non-Anglophone world, and this is an EU thread.
In Europe, the further left have always had their own parties due to the gift of proportional representation, and so have been capable of fighting their own corner. Unfortunately, it is the case that not that many people are very left wing. It's always a battle over the centre, because the centre plus one wing wins. You can constantly fight for just your wing, and you can leave your opponents free to win nearly all the elections. Take France. In the ~80 years postwar, France has had a left-wing French president for... 21 years. West/unified Germany has done slightly better, just shy of 25 years worth of left-wing chancellors out of the last 80. Italy is about 15 years of left-wing PMs. (Scandinavia is the exception, of course, but Scandinavia has unusual social traditions compared to the rest of Europe.) That's what we're talking about for the mainstream left. How do you think it would have worked out for the Communists?
Oh, yes, by the way. The far left in Europe have often been Communists. Real, genuine, actual, thank-god-the-Soviets-rolled-the-tanks-into-Czechoslovakia/Hungary Communists. They didn't always mix well with social democrats, because they were a bit iffy on the whole democracy, social freedoms, independence from Moscow sort of thing. (Plenty of these hoary old Cold War dinosaurs and ideals are still rattling around the European left.) Obviously, the centre and right loathed them. Sure, a few parties like Syriza have attempted to face the real world as a modern party, not that the real world was very kind to Syriza. Anyway, there they all still are: the names have often changed, they're not quite so Communist, but they are still full of egos, squabbling, schisms, temporary alliances and sometimes pretty horrible views. Still shit, still useless, still hated by the other 80-90% of the country. Their only chance is if the mainstream right AND left completely fuck things up, and even then the evidence clearly suggests that far right wins, not them.
There is the reality that the world is capitalist, and it's not kind to the left, because capitalists don't tend to be leftists. If you want to look at the countries free to pursue agendas really inconvenient to capitalism, you're looking at the likes of Cuba, Venezuala and North Korea. Not... pretty. Part of some of the British left's hostility to the EU was the idea that freed from its neoliberal clutches, they'd be free to enact all sorts of EU-rule busting socialist practices. Unfortunately, leaving the EU has instead - predictably - heavily suppressed the economy, so now it's even harder to pay for socialistic basics like universal healthcare and other social services. The price of policy freedom, eh? Someone's got to invest in production, grease the wheels, etc. and if you want that to be the government, then someone's got to get taxed or buy government debt. In the current world, these people are capitalists. The middle classes get twitchy at the idea of alienating them. So, make some concessions to your voters (and thus the inexorable pressure of international business concerns), or hand the keys of power to the right.