Film Theory: The Dark Reason Captain America REALLY Can't Lift Mjolnir
2. The Question Of Thor's Worthiness
In the first Thor movie, Thor is deemed unworthy and banished to Earth by Odin because his arrogant and impulsiveness lead to him shattering peace between Asgard and the Frost Giants and reigniting an ancient war. He's only redeemed when he seemingly chooses to give up his life to save Earth and is rewarded with the restoration of Mjolnir.
But there's more to Thor's worthiness than selflessness. If there weren't, Tony Stark and Cap would have been able to lift Mjolnir easily.
Thor's journey throughout the films takes him to the point of true worthiness only in Thor: Ragnarok, when he takes up his throne as the protector of Asgard and realises the importance of the preservation of his home's spirit, even after its physical destruction. Ironically, this comes AFTER Mjolnir has been destroyed, but it doesn't change his journey necessarily. And nor does it change why he's able to wield Mjolnir at all.
Arguably even more important to Thor's worthiness is his relationship with Loki, who he frequently forgives, despite the fact that he is far from good. He brings war to Asgard, brings war to Earth, replaces his father and brings war to Asgard again... And every time, Thor wishes to establish peace with him, because it is for the greater good of Asgard and the Nine Realms.
Even by the time The Avengers comes around, Thor isn't the same blood-thirsty man of war he had been before we met him in his first movie. As he qualifies in Avengers: Infinity War, he has killed 3000 enemies (before the battle of Wakanda), and given that we barely see him kill anyone up to that point (Hela's minions are already dead, really), it suggests he was particularly active in the field of killing.
Thor's arrogance when he is banished is the culmination of a life being bred by war and seemingly reveling in the "glories of war" rather than it being a necessity for the preservation of Asgard. From his banishment onwards, he is a completely changed character who no longer seeks war, mirroring exactly his father's move away from being an aggressor that's revealed by Hela's backstory in Ragnarok.
And that's where he differs from Captain America. Because Steve Rogers is a man fundamentally defined by war. And more than that, he actively seeks war. As Ultron said to him in Age Of Ultron: “Captain America, God’s righteous man, pretending you could live without war.”