Disappointed again
The left tends to be idealistic. Sometimes it segues into delusion (“working class revolution” … “democratic control of the economy by the masses”).
Therefore the left is prone to disappointment.
The Communist Manifesto was to the socialist left what Silent Spring was to the environmental movement. The Communist Manifesto appeared in 1848, during a year when there was, in fact, revolutionary ferment in much of Europe.
1848. So, by my count, there’s been 172 years of disappointment.
Here’s a chronicle of the latest:
https://jacobinmag.com/2020/11/extreme-center-war-on-the-left-socialism-organize-us-uk