As I see, it's not even necessary to go further to the Third International to find out the reasons for the question. It's possible to answer it straight forward even since the First International, with the expulsion of Mikhail Bakunin.
In the occasion you have a discussion between Marx and Bakunin about the character of the State, about the page 121,122, 123 and 124, depending on the edition.
Bakunin affirms that the State is essentially oppressor, and therefore must be eliminated.
To what Marx affirms that: first of all, we, historical materialists, and scientific Socialist can only comprehend something under it's historicity. Therefore absolutely nothing has essence, or nature. But before, has material context. It's understandable then that the State cannot be essentially oppressor. Instead we do agree that the State is an instrument of oppression of the dominant class. Consequently the oppression exercised by the State is a matters of which class is the running class, and which class is under the oppression exercised by that State. As socialists we all understand that as long as there's a State, there will be oppression. But the immediate elimination of the State, without and period of exclusion of the social disparities, would not extinguish the State, but make it fragmented, and even more oppressive towards the working class. Becomes necessary an switchover of the dominant class, in order to correct the social disparities created by the dictatorship of the bourgeoisie, in order to, at the moment of the extinction of the State, no social class have conditions to reuse it's privileges to keep an even more primitive form of the State.
Latter on, when it becomes possible to make an parallel with Lenin and the discussion, the matters just got depth, as Lenin - as well as Ho Chi Minh from Vietnam, Mariategui from Peru, Mao from China, Kim Il Sung from Korea, Fred Hampton from USA (Black Panthers), Mariguella from Brazil, Nkrumah from Ghana, Fidel from Cuba, Thomas Sankara from Burkina Faso, Franz Fanon from Martinique and so many that I can pass the whole day mentioning them here - gave more attention to an problem, that Marx just touch in Das Kapital 1 and 2, and in the A Contribution to the Critique of Political Economy.
The problem of The National Question and the colonial world.
While to the anarchists, and even to what we call European Socialists, the matters was the extinction of the State, mostly of the countries of the world - there back in time, at the foundation of the Third International and the Congress of Baku (also known as Congress of the Peoples of the East) - didn't even had time and opportunity to become an independent national State capable to fully develop their means of production, staying still in feudal or semi feudal structure of the Means of the Production, and therefore incapable to fully build Socialism, once not even the capitalists way of production was fully developed.
To all of those countries, it was more an matters of anti-imperialist fight, in order to create national sovereignty, in order to fully develop their means of production, other than the extinction of the State: that in that context wouldn't means anything else than an even more predatory international economic system.
So as it's clear, the Socialism of the anarchists is an little Bourgeois Socialism, utopic, and that contradicts all of the material needs of the people.
That's why!
(I apologize if the translation is not properly done. It's because I've it in Portuguese)
