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.