Just how communist are these far leftists. What groups were their antecedents?
I suspect that many of these leftist groups are more Marxist than Social Democrat (of old). The Soviets never stopped planting their operatives and money inside European parties and chosen individuals.
Remember, nearly every country in Europe had a Moscow-aligned (and directed, funded, staffed) Communist Party, with the biggest being the French (Marchand, etc) and then the Italian CP. Greeks were led by Theodorakis, Melina Mecourie, and the Andreas family. Portugal had Soares for a while. West German CP was an arm of the East German SPD.
Strong remnants of CPs in Hungary and Romania.
Maoist parties seem to be very small to almost non-existent.
Trotskyites have phone-booth parties with the exception of England where the Socialist Workers are strong.
Smaller CP’s in Scandinavia.
In other words, the Left is better organized, funded, and staffed with professional organizers than the liberal and conservatives. Too much fragmentation on the Center/Right which allows the united Left (with the Red-Greens aligned with them), to get Parliament seats and offices.