Because all of those people made serious tactical errors (such as allying with Stalin against each other) and just as importantly, none of them had a base of power within the party that could challenge Stalin's.

Stalin as general secretary made himself the de facto head of the party bureaucracy, with great deal of influence/power over appointments of people within the party hierarchy of the entire Soviet Union, which was the real source of power within the Communist party.

By contrast Trotsky's base of support was within the red army which sounded good in theory. However Communist parties with its ideological fixation on the French revolution was very keen on guard against "Bonapartism" which meant the army was always firmly subordinated to the party and Trotsky's supporters were largely removed in the 1920s. Zinoviev was the head of the Comintern as well as the Leningrad party apparatus but those positions turned out to be relatively powerless against Stalin's power over the rest of the country.