Barnes did take on the mantle of Captain America following Rogers’ apparent death in 2008, but Rogers eventually gets better and returns. For a while they were both Captain America, but eventually Bucky’s identity as the Winter Soldier becomes public and he’s put on trial. He is acquitted, but gives up the mantle because he thinks his checkered past taints the symbol.

A few years later, Steve grows old and retires, and specifically picks Sam as his replacement. Sam wonders why it wasn’t Bucky, and spends most of his time as Cap trying to prove himself.