He wasn't ruthless enough to keep his position during the power struggle that followed when Lenin got sick.

Then when Stalin took over, Trotsky decided it would be a great idea to suddenly find his backbone and aggressively push for "continuing world revolution" That would inevitably lead to the dismantling of the current Soviet state as it was at that moment. No one else wanted this. So Trotsky then got mad and began openly criticising Stalin for the lack of democracy in the Communist Party under his rule and his failure to have what Trotsky viewed as a viable economic plan for the future of the USSR. Remarkably Trotsky was tolerated as a nuisance for a hit before they finally got sick of him and exiled his annoying self after a drawn out propaganda campaign against him where he lost his important titles and jobs one by one.