Orwell wrote:
So, Dent, you support the idea of a worldwide communist revolution, right? But only in nations that have already passed through the capitalist stage of development. Out of curiosity (I am not very intimately knowledgeable of Marxist thought) why is it that he believed capitalism was necessary before socialism could be established?
Bloody hell Orwell the full answer to that question would fill a unit of study

blackelk is basically correct.
Marx basically saw the industrial revolution and the emergence of the working class as a chance for revolution. He saw the need for the full extent of capitalism to be apparent before this would occur,
He also believed that the revolution could not be successful in an agrarian setting, basically you need an educated working class that had suffered the worst of capitalism. As he wrote during the early 1850's
"With this general prosperity, in which the productive forces of bour-
geois society develop as luxuriantly as is at all possible within bour-
geois relationships, there can be no talk of a real revolution. Such a
revolution is possible only in the periods when both these factors the
modern productive forces and the bourgeois forms of production,
come in collision with each other... A new revolution is possible only
in consequence of a new crisis. It is, however, just as certain as this
crisis"
Marxism is not a set in stone idelogy, Marx himself expected it to evolve
"we do not confront the world in a doctrinaire way with a new princi-
ple: Here is the truth, kneel down before it! We develop new principles
for the world out of the world’s own principles. We do not say to the
world: Cease your struggles, they are foolish; we will give you the true
slogan of struggle. We merely show the world what it is really fighting
for, and consciousness is something that it has to acquire, even if it does
not want to"
He an Engels believed that by studying history and economics in a scientific manner you could predict the future course of capitalism, this is what modern Marxists do, we look at the historical consequences of the actions of the bourgeoisie to predict the future ramifications of their present behaviour.
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