Cowardice, bloated egos, lack of discipline and intellectual pettiness are often to blame, but that is not all.
It is now absolutely clear that the Western left lost patently and shamelessly. It has almost no power, it has no courage to fight or to take risks, and it counts on no real political following in Europe, North America, Australia or New Zealand. 'The masses', those proverbial 'oppressed masses', have lately been electing and voting in various semi-fascist populists, unapologetic right-wing demagogues, and mainstream pro-business brutes.
Entire Marxist 'theoretical certainties' have been collapsing in front of our eyes. Or at least they have been in the West.
To a great extent, what is now happening is absolutely natural. The European left betrayed as early as in the 1980's, by becoming too soft, too undisciplined, too cautious and too self-centered. It put pragmatism above the ideals. It rapidly adopted the lexicon of the liberal ideological establishment, complete with Western perceptions of human rights, democratic principles and political correctness. It ceased to be revolutionary; it essentially stopped all revolutionary activities, and it abandoned the core element of any true left-wing identity - internationalism.
Comment: Andre Vltchek suggests that the main issue with the left in the West is that it has given up its truly revolutionary (Marxist) roots. While it is true that the Western left has become a caricature of itself, the problem has much more to do with embracing postmodernist 'neo-Marxist' thinking, such as identity politics, rather than leaving behind revolutionary aspirations. After all, the world already saw what happened to Russia during and after its communist revolution - and who in their sane mind would like to see that happen again? Furthermore, who says that Social Justice Warriors would not be ready to spill blood and start a revolution in the name of their 'egalitarian' ideologies?
Vltchek is right about one thing: The illness of the Western left is most obviously manifested in its acquiescence of Western imperialism and the demonization of the Eastern rising powers and their allies, as well as other 'rebellious' nations in the 'periphery'.