Avengers: Age of Ultron had set up that they Captain America would be worthy of the wielding the Mjolnir. In the movie, Thor had held a small a contest where the Avengers had to pick up the Mjolnir, and everyone except Black Widow had tried to lift the weapon. No one could lift it this, but Steve Rogers was actually able to move the Mjolnir a bit. This had made Thor a little worried, but he knew that it was in his heart that something had been fishy.
In Avengers: Endgame, Thanos had almost killed Thor, and the Mjolnir had been lying on the ground. The hammer suddenly came flying, struck Thanos, and went back to the one who had called it-Steve Rogers. This scene was as good as Thor’s entry in the movie, Avengers: Infinity War in Wakanda. It was surely the best moment in the movie, Avengers: Endgame and the theatres had become colosseums again.
The first reason that Cap could lift the hammer is that Cap actually could lift the hammer in the movie, Age of Ultron, but he did not want to give Thor a shock. Thor was eventually shocked when Vision, and Hela and wielded the hammer.
However, the second reason is that Cap is a very righteous person. But since he had hidden the secret of Bucky killing Howard Stark, until Civil War, he was not worthy. He had shown signs of being worthy, but his worthiness had been unlocked when he revealed the truth to Tony and apologised.
Directed by Joe & Anthony Russo, Avengers: Endgame stars Robert Downey Jr., Chris Hemsworth, Mark Ruffalo, Chris Evans, Scarlett Johansson, Benedict Cumberbatch, Jeremy Renner, Don Cheadle, Tom Holland, Chadwick Boseman, Paul Bettany, Tom Hiddleston, Idris Elba, Danai Gurira, Peter Dinklage, Benedict Wong, Elizabeth Olsen, Anthony Mackie, Sebastian Stan, Gwyneth Paltrow, Josh Brolin, Pom Klementieff, Karen Gillan, Dave Bautista, Zoe Saldana, Vin Diesel, Bradley Cooper, Chris Pratt, Paul Rudd, Samuel L. Jackson, Cobie Smulders, Tessa Thompson, Evangeline Lilly, Michelle Pfeiffer, Michael Douglas, Tilda Swinton, Ty Simpkins, Ava Russo and Brie Larson.
Here’s the new synopsis of Avengers: Endgame:
“Thanos snapped his fingers in Avengers: Infinity War and half the life in the universe instantly dissolved. Now a ragged group of surviving warriors, including Captain America, Black Widow, Iron Man, Thor, and the Hulk works to counteract the Mad Titan’s genocidal action. Since our heroes couldn’t stop Thanos the first time, what makes them think they can win in a rematch? The answer may be in overwhelming power, thanks to Captain Marvel, or in the tiniest spaces between atoms accessible to Ant-Man. Avengers: Endgame will rewrite the Marvel Cinematic Universe as we know it. When it’s all over, nothing will be the same.”