It is mainly due to the time period of which his books Capital, Volume 1 and the Communist Manifesto were written. The late 18th and early 19th century was a time of significant progress in Europe due to the industrial revolution. Urbanization and food production were at historic highs. However the set of demographic values with those of the industrial revolution also put economic disparity at the forefront of social issues at the time. The conditions in the factories were horrible, but being unemployed even more so, while the Capitalist reeked the benefits of this transition period. The period was also a time of revolution and ideas. The French Revolution, Napoleonic Wars, and the American Independence all had seeds of which the common collective took a stand against a centralized elite. A seed the sewn across Europe. Further more the failed revolutions of 1848 marked a superiority of the upper class against the common class of workers.Karl Marx's ideas were built on those foundations. Foundations that asked for a system of equality and more importantly called out the inevitable exploits of Capitalism. He thus gave a voice to the working class- an empowering idea that they can overthrow the elite.
Marx also wrote his Manifesto, along with Engels, in a simplified language of which the working class was formed. All in all it was a perfect storm for Marx's ideas to take hold. Hence why the ordinary workers of Europe, especially in Continental Europe where the Napoleon Wars made conditions even more so to the radical ideas of Marx.
Hope that helps!