This is a picture of Vladimir Lenin and Joseph Stalin.
When Lenin died, there was a natural need for a next ruler to govern Russia. Leon Trotsky, at the time, seemed to be the natural successor, but in the end, Stalin won and rose to power. Three key reasons for Stalin's winning were: luck, policy, and Trotsky himself.
Luck is considered to be a key reason for his being favored by others, and Lenin's last will and testament had something to do with it. Lenin's last will and testament contained very critical judgements of Stalin and many other officials. Since the officials feared that this would affect their chances in becoming the next ruler of Russia, it was never published.
Policy was another reason because ,at the time, there were two prevalent issues being discussed. And, this were: Focusing efforts and resources in other countries and let them stage their own revolution versus Focusing on Russia to make the model Communist Russia and Continuation of N.E.P. versus Stopping of N.E.P.
Stalin was able to make use of these issues to his advantage and got support from the party members that sided with him.
And lastly, Leon Trotsky himself was a key reason because he underestimated the capabilities of Stalin. Because of this, he took it easy and this decision backfired on him because Stalin, in the end, rose to power and he was forced to exile. The efforts that Stalin put were able to convince the others that Stalin was more like Lenin.