A little outside of my wheelhouse, but I do have the good fortune of having access to Raymond Carr's classic work on the Spanish 19th and 20th centuries: Spain, 1808-1975, Second Edition, which is part of the excellent Oxford History series on Modern Europe.

First, a short note. Anarchism did have an important draw in Russia. As it did in parts of Italy, as well as the case in point: Spain. Many key anarchists in the US, for that matter, were from Russia and Italy. However, it was certainly strongest in Spain out of all of them.

If you read the autobiography of Russian born Anarchist Emma Goldman you'll find many of her closest collaborators were also foreign born, many Russian.

Eric Hobsbawm's analysis of the issue in his book The Age of Capital is essentially that anarchism had a special draw in countries which were still going through the birth pains of modernization, but had yet to emerge through to the other side. In other words, they suffered both the backwardness of aspects of the Ancien Regime as well as the economic problems specific to Capitalism. As such, the anarchist slogans against the State appealed to much older traditions of peasant anger against the State and its exactions as well as anger at landlords. Hobsbawm also takes pains to point out that as an ideology, anarchism also had important strains of bigotry (such as anti-semitism and xenophobia) which made it familiar and attractive to those tendencies among peasants and laborers in backwards areas in Europe. As such, it appealed to peasants and rural proletarians (who wanted to just work their land and receive the benefit, without rent for landlords or heavy taxation), as well as artisans and semi-skilled workers who eschewed the factory system.

I will, however, note that Hobsbawm's statements on this topic are not entirely neutral, as he was a Communist and was not necessarily neutral on that topic. My own reading of Emma Goldman's autobiography (she was Jewish) shows no signs of major issues with xenophobia or anti-semitism, but it has been a few years since I read it and her personal experience may not reflect bigger issues that Hobsbawm was referring to there.

In essence, Hobsbawm sees it as a sort of half-way ideology for social and political change in societies still in transition towards real modernity.

Carr's explanation of anarchism's hold in Spain is somewhat similar, in that he points out how the suspicion of the State, anger at the failure of traditional institutions (such as the Church) to take the worker's side, and the major economic woes of rural proletarians, peasants, and semi-skilled workers still being pushed towards factories. What really boosted the movement in Spain was a vicious cycle involving State repression and terrorism.

Basically, more moderate anarchists would push for labor actions (walk-outs, etc.) to improve working conditions. The State would stick them in jail and brutally suppress their attempts to organize. This made more radical elements within the movement decide that terrorism was okay, since non-violent action would just be suppressed. These acts, like bombing Church processions and assassinating political leaders, then alienated much of popular opinion and gave the State even more reason to brutally crack down on the movement as a whole (though the terrorist elements were a minority). The crackdowns would hurt innocent people and radicalize those around them.

In the specific case of the CNT (Confederacion Nacional de Trabajo), their anarcho-syndicalist bent made them "a bridge to the older unionism that anarchism itself had often lacked". Again, its radicalism was, in Carr's view, pushed on it from the outside "by the intransigence of employers and the erratic governmental repression." (p.446)

In this way, the very strength of conservative elements in society also helped pushed workers towards anarchism, since moderate means were often futile and repression brutal. This same repression also helped radicalize them towards hard line anarchism once they joined the movement.

Hope the answer helps. A bit outside of my wheelhouse, but definitely an interesting subject.