Avengers: Endgame saw Steve Rogers/Captain America lift Mjolnir during the final battle against Thanos, proving himself worthy of it. But this wasn't always the case as seen in Avengers: Age of Ultron when he tried and failed to pick it up. So what made the super soldier unworthy to lift Thor's hammer and what did he do to finally be granted the power to use it eight years later? Directors Joe and Russo explain that Steve has always been able to pick up the mystical hammer, but the real reason may all boil down to his close relationship with Bucky (Sebastian Stan).
There's no denying that Steve has always been a good man. Long before he's picked for the super soldier program and become Captain America, he has always had the core values of a principled man - in fact, this is the very reason he was chosen for the scientific project amidst more physically capable candidates. He's the Avengers' moral compass, and he's proven time and time again that he is the textbook definition of a hero. Despite all this, Steve is not perfect, as every hero in the Marvel Cinematic Universe, and has made questionable decisions throughout the years. Perhaps one of the biggest ones was to intentionally hide the fact that he knew about Bucky murdering Tony Stark/Iron Man's (Robert Downey Jr.) parents, Howard and Maria Stark.
This was the personal conflict that ultimately drove Steve and Tony apart, leading up the break-up of the Avengers in Captain America: Civil War following their ideological disagreement regarding the Sokovia Accords - a contract that would put the superhero team under the supervision of the United Nations. Interestingly, Captain America was against signing the agreement, while Iron Man was all for it. But both sides have valid points for justifying their stance about the Accords, but Steve lying about Bucky's involvement with the Starks' death was just plain wrong.
Captain America discovered this tidbit during the events of Captain America: The Winter Soldier, when he and Natasha Romanoff/Black Widow (Scarlett Johansson) were diving deep into how HYDRA was able to quietly grow inside S.H.I.E.L.D. in the last few decades without being caught. Steve didn't see the tape of Howard's murder until Baron Zemo (Daniel Bruhl) played it in Captain America: Civil War, but during his encounter with computer-operated Zola, it was heavily implied that the Starks' accident was planned. Considering when that tragedy took place, coupled with the realization that Bucky was brainwashed to become the Winter Soldier, it didn't take Steve long to put two and two together.
Steve kept this information for two years, although he was consistently in contact with Tony, both being part of the Avengers. He was keeping this massive secret during the party in the Avengers Tower where everyone was given the chance to pick up Mjolnir. Sure, he was able to move it ever so slightly because this lie didn't cancel out the fact that deep down, he has morals that no one else in the group can rival other than Thor and later, Vision. But that doesn't mean it's not still wrong for him to intentionally hide the truth to protect Bucky. When he was confronted about this in the bunker, he even tried to deny direct knowledge about this, until Tony saw through this and asked him again, to which he fully confessed.
But with that out in the open for years by the time the events of Avengers: Endgame rolled out, Steve no longer has anything in his conscience. He's also patched things up with Tony as they both admit that resentment will bring them nowhere, especially given their mission to bring back everyone who was wiped out by Thanos' (Josh Brolin) snap in Avengers: Infinity War. Given all that, by the time he called onto the magical hammer, it came to him, proving of his worthiness to wield the Mjolnir.
