Well there’s a number of reasons here.
The first to note is that communism is not solely a theory which applies within an industrialised society. Within the less industrialised countries of China and Russia, communist parties also offered solutions to the rural peasantry, which was important in areas with a relatively small urban working class.
Further, to some extent such societies are weaker than highly industrialised nation states to revolution. Within industrialised states many individuals of the potential revolutionary classes may be convinced to benefit from capitalism instead of seeking to destroy it, with forces such as nationalism counterbalancing revolutionary urges. As a result, such states would often be able to claim solidarity between the people’s based upon a sense of common national belonging which was able to supersede class identities. In the First World War this was the case in Germany, where the socialist parties mostly supported the war at the beginning, much to the despair of internationalist revolutionaries abroad such as Trotsky, Kollantai and Bukharin, who had hoped that an international solidarity between the proletariat peoples would overcome nationalist sentiment. Russia and China with their vast multiethnic empires found nationalism a far less effective tool for the reactionary state.
In addition, such states often benefited from stability which had several side effects. One was that it alienated the revolutionaries from the other reformist groups. In Tsarist Russia and nationalist China, liberals could often convince themselves that groups such as the CCP and Bolsheviks were the lesser of two evils, and would decide to support them against autocracy. Im contrast, in societies where the middle classes were not so alienated from the establishment they would often feel threatened by the prospect of revolution and support the government. Further, developed states were often both more efficient and organised in governance, allowing far more effective policing and counter revolutionary activities. Such conditions made revolution somewhat easier within societies which were agrarian based, and lacked the more centralised bureaucratic rule of the modern industrialised nation state. In contrast, the essentially feudal societies of Russia and China were chaotic to say the least, and with the incredibly pervasive nepotism, cronyism and corruption, they were often very poorly led and organised. In contrast, the meritocracy of revolutionary groups could often lead to the promotion of talented individuals. In Tsarist Russia, generals and officials were often promoted by the Tsar based upon loyalty and popularity rather than skill. In contrast the Bolsheviks were rather more willing to advance an individual based upon merit, it is certainly interesting to note that many of the Tsar’s best generals (such as A.A Brusilov) joined the Reds during the civil war, and that many of the Red Army's best generals had been NCOs in the Tsar’s army (i.e. Zhukov, Chapaev, Rokossovsky). We can observe a similar disparity in the Chinese civil war between the forces of the CCP and the GMD.
Further, communist theory seemed to offer a route for a degree of independence for colonised peoples, who had often found themselves disappointed by the hypocritical execution of principles of self-determination by the likes of France, Britain, and the USA. In Vietnam for example, it was after the disappointment of Versailles that communism began to spread. The same is true of China, where the May Fourth Movement (a reaction against the perceived betrayal of China by the Western Allies) played a crucial role in the rise of the CCP as a political force. Further in China the CCP has made great propaganda use of its stance against western imperialism (including Japan) and frequently claims it was Communism which ended the ‘century of Chinese humiliation’ by foreign aggressors. For oppressed colonial society such rhetoric is moving.
Ultimately the point is that communism does not apply solely to capitalist societies, in fact it can appeal equally so to the serf and the peasant, living in horrible conditions of poverty that would be recognisable to their ancestors hundreds of years before. The grievances of the poor farmer dominated by the hereditary nobility are often similar in character to that of the urban worker. These agrarian based societies therefore had very large populations who were therefore willing to support communism, in fact possibly more so than in capitalist societies, and further the political and structural conditions of those societies often made it easier for communism to seize power.
So this is a bit of meta question which is somewhat comparative in nature, so I’ll provide some sources that give an insight into some of these specifics although with a bit of a Russian and Chinese bias as expressed above.