The theory was based on some image models, which were understood easily by the population of those times. As we know, it is not possible to find the right importance of an element or relationship on an image model. Thus, an image model can be tailored to prove anything, based on external reality. Such a method doesn't build a theory, but an ideology. Ordinary people easily understand an ideology, because it sounds good, and is able to answer any question.
When Marx's theory appeared, the control of the economy was based on short range symbolic models. So, long-range problems were not predicted. The lack of stability of the society in that period was an effect of this problem. Later, society advanced to take into account the long-range problems too and so, social-democratic movements appeared.
1878 BC: Jehovah's Witnesses In time, as the capacity to make and operate symbolic models increased, the "classical" Christian religion was under pressure to change to meet the new requirements. It resulted in reform movements in the frame of the Christian religion. Instead, Jehovah's Witnesses make a new model starting from scratch.
1900 BC: Max Planck introduces Quantum Mechanics
Quantum Mechanics is the first purely symbolic model associated to external reality. Such a model is "pure", because it cannot be translated or associated with any image model. This is a very high step in the evolution of the brain (level 5 of evolution).
Based on data, it seems that Max Planck understood from the beginning that his theory was good, but he was blocked by the fact that imagination does not work in understanding it. It seems that Albert Einstein pushed him to publish the theory.
Quantum Mechanics cannot be translated to image models. If someone forces such a translation, aberations or logical contradictions occur. The best known problem is the nature of light. Quantum Mechanics predicts that light is both wave and particle. This situation cannot be understood based on image models.
Quantum Mechanics pushed forward the evolution of society on a very high technological level (e.g. the transistors and the lasers are the basic components to build computers).
1905 BC: Albert Einstein, the theory of the relativity
Einstein starts from Euclid's Geometry and Newton's Mechanics to make a new theory, to extend the knowledge to atomic and sub-atomic levels, and to the far Universe. The "marriage" between Quantum Mechanics and the theory of Relativity builds the main tool to understand sub-atomic "Universe".