They became trapped in their own unwillingness to accept reality and give up their failed ideology.
I think Horowitz explains it best, but I will try to synopsise:
Marxism claimed to be scientific, and seemed reasonable when it said capitalist society would concentrate wealth in an ever smaller circle of hands until the workers revolted.
That never happened and had to be explained by a gradual widening of the gap between rich and poor, even if the poor did better in an absolute sense, resulting in a revolution.
That never happened, so a theory was devised where the rich nations got rich by stealing from the poor nations, so eventually the poor nations would revolt.
You can see how this gets further and further from reality as the leftists try to reconcile reality with a failed ideology. It gets us to where we are today, where the left supports fanatical Islamic fascists because they oppose the United States. This has to be the "Revolution" because the United States is rich and the Islamic countries are poor. Therefore the U.S. is evil and the terrorists are freedom fighters, a la Micheal Moore.
Sad, but true.