Why do Marx and Engels feel a proletarian revolution is necessary?
In The Communist Manifesto (1848) Marx and Engels argued that history should be understood as a history of class conflict between
Freeman and slave, patrician and plebeian, lord and serf, guild-master and journeyman, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on an uninterrupted, now hidden, now open fight, a fight that each time ended, either in a revolutionary reconstitution of society at large, or in the common ruin of the contending classes.
In a point Marx would elaborate, expand upon, and defend in his subsequent writings, particularly Kapital, he and Engels argued that the emergence of capitalism had led to a crystallization of the social classes into the bourgeoisie, which he defined as those who controlled the means of production, and the proletariat, those who labored for them. The bourgeoisie, they said, had amassed great wealth and power by the ruthless and systematic exploitation of the working classes:
The bourgeoisie, wherever it has got the upper hand, has put an end to all feudal, patriarchal, idyllic relations. It has...left remaining no other nexus between man and man than naked self-interest, than callous “cash payment”...In one word, for exploitation, veiled by religious and political illusions, it has substituted naked, shameless, direct, brutal exploitation.
They argued that this process of exploitation would increase the size of the proletariat, as it drove more and more artisans, small farmers, and other self-sufficient men into wage labor. In other words, the wealthier and more powerful the bourgeoisie became, the larger and more disenfranchised ("alienated" in Marxian thought) the proletariat would became. The process would accelerate and intensify as mechanization eliminated the demand for skilled labor.
Mass production, a result of the bourgeoisie's relentless drive for wealth, would have two unintended consequences for the bourgeoisie. It would concentrate their numbers in urban regions, where manufacturing centers would house millions of disenchanted workers living under appalling conditions. Engels had himself witnessed, and written about, these conditions in Manchester, England. Second, the drive for profits would create overproduction, an "epidemic" that would lead to repeated and worsening economic downturns. Eventually, and inevitably, the working class would rise up and destroy their exploiters, and the Communists, Marx and Engels said, would lead the way:
The immediate aim of the Communists is the same as that of all other proletarian parties: formation of the proletariat into a class, overthrow of the bourgeois supremacy, conquest of political power by the proletariat.
It should be noted that Marx and Engels wrote in the midst of a wave of unrest that shook Europe, including a major revolution in France. They interpreted these events as the beginning of a massive working-class uprising, though they also thought that the process of creating a fully developed proletariat, a condition necessary for revolution, would take time in many countries, especially those that were not fully industrialized.