Specifically I think about how the Russian Empire at the time of the October Revolution wasnt as industrialized as the rest of europe. Or how China wasnt really industrialized at all with more examples in places like Vietnam and Korea. I Hesitate in including Eastern Bloc nations like Poland and Romania as they were sort of "coerced" into being communist states by the USSR.
I answered a similar question a few months ago which talks about Marxism's application in the Russian Empire.
The short answer is that the October Revolution was almost entirely orchestrated in the nation's industrial capital, Saint-Petersburg (then called Petrograd) so Marx was right to a certain extent. Despite this, very few early Marxist thinkers (including Lenin himself) predicted that the socialist revolution would occur in Russia because of its largely agrarian society-- as you mention.
Lenin combated this diversion from Marxist orthodoxy by implementing certain language like 'rural proletariat,' to bring those outside the actual cities under the purview of his revolutionary aims. When that wasn't totally effective, the Bolsheviks just reverted to type and socialized the countryside by violence and coercion. There's several responses in that question beyond the initial answer that talk about this in pretty excruciating but I'm happy to answer any follow-up questions you might have.