It essentially boils down to the fact that many people were not conscious of the problems of their governments and Capitalism's failures until major wars happened, World War One in particular provided such terrible conditions that Germany and The Russian Empire had massive revolutions, and after that there's the fact that the success of the Russian Revolution had many see the establishment of Marxist Leninist forms of government as a way for Socialist ideas to be facilitated and spread, which inspired many to have revolutions following the USSR's form of government after Lenin died(whole other topic) most of these places such as Cuba, Angola, Mozambique, North Korea prior to Juche, China, and Vietnam inspired many to take up arms against oppression as well. TL:DR- Ww1,Success of Russian Revolution, Increase in awareness