Marxism has proved adaptable to power-seekers. Of course, Marxism is not the only ideology with that property. Finally, we need to ask why so many people stuck with communism even after seeing its failures and crimes. Some were implicated in the crimes and profited from them. Others hoped that the Soviet Union would get better in spite of seeing that it wasn't.
Humans have great capacity for wishful thinking.
Many people stayed in the communist movement out of fear of the consequences of quitting. The Party launched rumor campaigns in which defectors were accused of being drunks, wife beaters and embezzlers. A person might see the crimes of the Soviets but still accept the Party propaganda about the evil nature of the FBI and American police departments. Currently the green ideologies have some of the same properties as Marxism, but they don't pretend to give a comprehensive view of human affairs. They become more motivating to their adherents when combined with some of the leftovers from Marxism that provide attractive theories about whose fault it all is.