Why did Marx believe that mass industrialisation was a prerequisite for the proletarian revolution?
From his Communist Manifesto, Chapter 1, Marx believed that the industrial revolution brought about the existence of the proletariate, from the crises of Capitalism always needing new markets, greater exploitation of workers for more profits.
Modern bourgeois society, [...] It is enough to mention the commercial crises [...] In these crises, there breaks out an epidemic that, in all earlier epochs, would have seemed an absurdity — the epidemic of over-production. [...] And how does the bourgeoisie get over these crises? On the one hand by enforced destruction of a mass of productive forces; on the other, by the conquest of new markets, and by the more thorough exploitation of the old ones.
But not only has the bourgeoisie forged the weapons that bring death to itself; it has also called into existence the men who are to wield those weapons —the modern working class —the proletarians. [...]
Marx didn't believe the previous classes, or class struggles were long lasting. Eventually, society would have to collapse to socialism with no more private property rights. Instead, the proletariat would do away with all national divisions for the ever increasing size and power of the union.
Of all the classes that stand face to face with the bourgeoisie today, the proletariat alone is a really revolutionary class. The other classes decay and finally disappear in the face of Modern Industry; the proletariat is its special and essential product.
The lower middle class, the small manufacturer, the shopkeeper, the artisan, the peasant, all these fight against the bourgeoisie, to save from extinction their existence as fractions of the middle class. They are therefore not revolutionary, but conservative.
Modern industry and its division of capital and labor, places ever increasing pressures on labor to reduce costs. The opening of new markets removes the isolation of laborers from one another. The proletariat, as the lowest class, sinking further and further into poverty has no choice but to revolution.
Hitherto, every form of society has been based, as we have already seen, on the antagonism of oppressing and oppressed classes. But in order to oppress a class, certain conditions must be assured to it under which it can, at least, continue its slavish existence. [...] The modern labourer, on the contrary, instead of rising with the process of industry, sinks deeper and deeper below the conditions of existence of his own class. He becomes a pauper, and pauperism develops more rapidly than population and wealth. And here it becomes evident, that the bourgeoisie is unfit any longer to be the ruling class in society, and to impose its conditions of existence upon society as an over-riding law. It is unfit to rule because it is incompetent to assure an existence to its slave within his slavery, because it cannot help letting him sink into such a state, that it has to feed him, instead of being fed by him. Society can no longer live under this bourgeoisie, in other words, its existence is no longer compatible with society.
The essential conditions for the existence and for the sway of the bourgeois class is the formation and augmentation of capital; the condition for capital is wage-labour. Wage-labour rests exclusively on competition between the labourers. The advance of industry, whose involuntary promoter is the bourgeoisie, replaces the isolation of the labourers, due to competition, by the revolutionary combination, due to association. The development of Modern Industry, therefore, cuts from under its feet the very foundation on which the bourgeoisie produces and appropriates products. What the bourgeoisie therefore produces, above all, are its own grave-diggers. Its fall and the victory of the proletariat are equally inevitable.
Marx, was of course wrong. The working class, never became poorer and poorer. After 50 years of failed predictions, they never had to revolt 10:20
Oh, I didn't notice there was an almost simultaneous answer (you win, by a minute ;).