One of the best moments of Avengers: Endgame came when Captain America, played by Chris Evans, was worthy enough to lift Thor's hammer during the final fight with Thanos. It was such an epic reveal that people cheered in the theater when I saw the movie, myself included. Steve Rogers/Captain America's journey in the Marvel Cinematic Universe has been one of the most interesting to watch, and seeing him lift Thor's hammer was a stunning conclusion to his arc. However, the moment left some fans wondering why Steve wasn't able to wield the weapon in prior battles.
ComicBook.com recently hosted a quarantine watch party of Avengers: Endgame, where the film's writers Christopher Markus and Stephen McFeely were asked why Steve didn't lift the hammer during the Avengers Tower party scene in Avengers: Age of Ultron. According to Markus, it had to do with Cap's best friend Bucky, a.k.a. the Winter Soldier. Markus said that Cap couldn't lift the hammer because he knew that the Winter Soldier had killed Tony Stark's parents. However, this information doesn't come to light until Captain America: Civil War, so Steve might have been burdened with the secret and thus unworthy to lift the hammer.
There have been other opinions on why Steve didn't life the hammer until Endgame. As reported by ComicBook.com, Marvel Studios executive Louis D'Esposito has his own view on the matter. "If you remember from Ultron, they were all sitting around in the Avengers complex in Manhattan, and there's a party, and they're all a bit inebriated, and they're loose, and they're having fun, and they're all trying to pick up the hammer," D'Esposito said. "It's Captain America's turn to try, and you look over to Thor's face, and he says, 'I think he might be able to do it,' but Cap doesn't pick it up. But Cap could've always picked it up. He didn't want to at that point because it would've not been right."
No matter the reasoning, watching Cap lift Thor's hammer was incredibly satisfying. Rewatch Avengers: Endgame, along with tons of other fun titles, with a subscription to Disney+ here.