Karl Marx was a very influential thinker because he had a very provocative idea at a time that was ripe for that idea to take hold.
Marx wrote his most important works in the late 1840s, the 1850s, and the 1860s. This was a time when Europe was industrializing. Society was changing as more and more people left the countryside and came to cities to work in factories. Society and the economy also changed as more people had to stop being artisans and had to work in jobs that paid less and required less skill. This was a time of great social and economic change. Workers were unhappy as they came to have less autonomy and lower standards of living.
Marx became so influential because he arrived at that particular time with a theory that purported to explain why workers were justified in their unhappiness and what they could do about it. Marx’s theory argued that capitalism consisted of unfair exploitation of workers. It explained that the capitalist class used a variety of methods to keep the workers subjugated. It laid out the idea that workers could unite and engage in class struggle against the capitalists. It promised that workers would inevitably win this struggle and would thereby create a new society in which workers would no longer be exploited. This would be a utopian society where each person gave according to their abilities and each person got things according to their needs.
Karl Marx arrived on the scene at a time when workers were very unhappy and he promoted a theory that told them they were right to be unhappy and explained how they could eventually overthrow the system that oppressed them and create a better society. Therefore, it is no surprise that he became such an influential thinker.