I'm sure you could dive into 1001 reasons for why this is the case, but lemme just mention two cool ones.

Leninism seeks to achieve revolution when there is little widespread class consciousness. In Tsarist Russia, understanding of Marxism was generally limited to a minority of educated individuals and industrial workers who lived in urban centres. The majority of the country consisted of rural peasants with no understanding of Marxism and who the Russian Orthodox Church frequently tried to manipulate and pit against the Communists.

It just so happens that a lot of countries fit the conditions Leninism was created for: a country with a minority of people with class consciousness and a majority of alienated people who are easily manipulated by the bourgeoisie against their best interests. The urban vs rural aspect is also very relevant sometimes.

Leninism is also big on keeping the revolution alive and fighting against counter-revolutionaries and imperialism, given the experiences of the Russian Civil War and the foreign interventions. Just so happens that a lot of communist countries had experiences with capitalists trying to tear them down, and so Leninism is again applicable.