You can find more detailed analysis on other threads but in short Trotsky was kind of an unlikable prick on a personal level and the Bolsheviks had a deep-seated paranoia of some asshole pulling in Napoleon and derailing the revolution.Why is it so unlikely? IMO if he had made good decisions and had a bit of luck he could have won his fight against Stalin and co. I didn't say he was a warmongering, just that he was less cautious on foreign policy than Stalin.
Besides there are other candidates for potential leadership roles in the Soviet Union that aren't just Stalin and Trotsky.