The first factor that allowed Stalin to win was his prowess at building a network of supporters beneath him in the party apparatus. Stalin’s most important post for this purpose was the post of general secretary of the Communist Party. From that post, Stalin had the power to appoint many party functionaries. By doing so, he was able to create a network of party workers who owed him and who were sympathetic to him.
The second factor was Stalin’s ruthlessness. Stalin was willing to do whatever it took to gain power. He was willing to change his ideas based on the opponent he was facing. He was willing to say one thing to defeat one opponent and then the opposite to defeat a different opponent. He was willing to destroy people who had helped him if he felt they were becoming a threat.
Stalin, then, was both ruthless and very good at doing the little things that built support for him in the Soviet system.