"Marxist interpretation of the 20th century is much more convincing in both its explanation of World War I (imperialism as the highest stage of capitalism) and fascism (an attempt by the weakened bourgeoise to thwart left-wing revolutions). But Marxist view is entirely powerless to explain 1989, the fall of communist regimes, and hence unable to provide any explanation for the role of communism in global history. The fall of communism, in a strict Marxist view of the world, is an abomination, as inexplicable as if a feudal society having had experienced a bourgeois revolution of rights were suddenly to “regress” and to reimpose serfdom and the tripartite class division. Marxism has therefore given up trying to provide an explanation for the 20th century history."
It's true that the dogma of the official communist parties has no explanation for this, but their theory degenerated and became distorted by the need to justify all the twists and turns of USSR foreign and domestic policy. A genuine Marxist view of the 20th century is that the world revolution began in 1917 in Russia, the "weakest link" of capitalism, but was not able to be completed in the industrial centers. There were a number of reasons for this, for example the growth of reformism in the German SPD, but the result is in reality a failed revolution that won a few outposts and held on to them for 70 years. As a useful example, the Paris Commune is another example of a failed revolution, although it only held out for a few months. It's not a Marxist view that a single country can "go socialist" in a world dominated by capitalism. So there was no "regression" back to capitalism.