People thought that Trotsky would be the next leader because he seemed to have the characteristics that people think leaders should have. Trotsky was a brilliant person. He was a strong political thinker. He was able to translate his political thought into inspiring speeches and writings. He had also proven his leadership ability as he had organized the Bolshevik Revolution nad had led the Red Army in the Civil War.
By contrast, Stalin did not seem like he had the right attributes. He was not Russian. He was not someone who seemed remarkable and brilliant. Instead, he seemed more like someone who was good at getting things done, not at leading.
Trotsky seemed like an ascendant talent while Stalin seemed more like a “work horse” who would never be able to steal the spotlight from the more exciting “show horse.”