I don't see how Russia isn't part of the West, they were founded by Vikings just like the rest of the Faustian Culture and developed along very similar religious, political etc lines for hundreds of years. I haven't seen an explanation for why Russia isn't part of the West but Spengler might explain it somewhere, I assume it's just Catholic vs Orthodox stuff or something.

I think a lot of the 17th-20th century discourse on Russia is down to exoticism, to be honest. As far as I am aware, it starts with French Enlightenment thinkers who used Russian serf communes and juxtaposed the "natural harmony and liberty" of the common people against oppressive and obscurantist Western European absolute monarchies. Then when the bourgeois revolution actually set firm roots in the West, Russia transformed into the religious, obscurantist absolute monarchy, but this time the image was also bolstered with ideas of "passive orientals", "Oriental despotism", "resistance against dynamic Western progress" etc. Finally, during the Soviet period, people on the far right with an interest in Russia ended up synthesising these two views and mixing those ideas about "oriental passivity", "brutal Oriental despotism" with a renewed, selective emphasis on the more social, communitarian, collectivist traditions of Russia. You can find this in various sources - I think Stoddard took a similar position in order to claim that the Russian revolution was simultaneously an "oriental revolt against civilisation" and the assertion of an anti-Russian, materialist and technophilic Bolshevik movement.

In my view, this type of opinions are usually just a result of polemics and the diffusion of polemical ideas within a broader political subculture. A lot of them are tendentious and deliberately uncharitable, so I can only see them as polemics. I've seen some of this stuff with Evola as well - he very rarely makes mention of Imperial Russia. I think the only time he mentioned it was in his commentary on the Protocols, when he briefly described its role in Europe as a defender of rightism. It's the same with Orthodoxy - from what I remember, he affords it only two lines, and ends up dismissing interest in it despite also acknowledging that it's a traditional religion which even includes some initiatic elements. He gives more attention to the Soviet Union and describes it precisely as a kind of anti-Europe, something which has defined itself against Europe and something which Europe should also define itself against. I think at least in the case of Evola, and perhaps also in that of other people like him, the anti-Russian stance may have been affirmed as a kind of practical polemic in an attempt to help mobilise Westerners and raise their political and moral strength. Otherwise, the only other explanation I have for his low interest in Russia is a possible lack of sources on Imperial society. I would have thought that Evola might have an especially positive opinion of it, because of the Russian claim to being Third Rome, the fact that Russia upheld the ideal of a divine imperium and a holy nation all the way up to the 20th century etc. Despite that, I can't find any positive mention of Russia. Ultimately, I think the anti-organic character of the Soviet Union was probably the reason why Evola presented Russia as an anti-Europe and ignored not only any positive elements of traditional Russia, which, admittedly, were already lost at the time, but also ignored the positive elements and the possibilities offered by revolutionary Russia as well. For example, in one essay on the Neue Sachlichkeit - probably the most difficult text of his that I have ever read - he attempts to outline an ethos suitable for modern men, and explicitly compares the Nordic Neue Sachlichkeit with Soviet ideas of cold realism, but ends up attributing a completely opposing character to these two sets of ideas. In my view, he seems to do this mostly for polemical reasons. Naturally, I can't claim to know what Evola's thought process was as he wrote his articles, so these are all just my own interpretations.

Things also become a bit more complex when we shift away from holistic thinkers like Evola and look at more general trends. There were several common types of arguments used to attribute a special, "oriental" character to Russia, but some were used more often by critics from different countries. For example, the heavily modernist character of Atlantic states often meant that critics from those states chose to criticise precisely those elements of Russia which had a genuine traditional character shared also by pre-modern Western states. In Germany, on the other hand, critiques tended to be more essentialist in nature, for various reasons.

To briefly return to the original point, I think Spengler claims that High Cultures develop in relation to space and are in a way based on the environment around the people of the culture. Spengler posits that the Western Faustian worldview is the three-dimensional worldview of "infinite possibilities", whereas the Russian one - based on the endless steppes - is that of "the world as one-dimensional plane". To be honest, I have no idea what this is supposed to mean, what systematic values these assertions hold, or how Spengler arrived at them. Admittedly, I have only skimmed Spengler's work, but his ideas do not seem especially objective or persuasive to me.