Theory That Worthiness to Wield Mjolnir Evolves
First evolution, must be willing and able to swallow pride and forsake glory for the greater good. Thor 1, he has to learn humility and the value of peace.
Second evolution, be willing and able to turn on those you love for the sake of the greater good. Avengers, Thor has to stop trying to reason with his brother and stop him.
Third evolution, must be able to put the good of others above the good of rulership/government. Avengers 2, Steve can move Mjolnir but not wield it. This theory means that it isn’t until Civil War that Steve can actually wield Mjolnir. He had to fully break with government and law to realize what’s right.
Fourth evolution, to sacrifice the self, core values, or morality for the greater good. Avengers: Endgame. Here Thor is still worthy and Steve is fully worthy deapite drastic changes. Thor is constantly working through loss, pain, regret, trauma and failure. To the point that he just wants to rest and not worry about anything anymore. He puts all this emotional baggage and damage aside to become the hero once more.
Steve has been preaching to people that it’s time to move on, look on the bright side, pick up the pieces. Instead he time travels. He goes back to the past to fix what was broken. To be woth the woman he “loves”. He goes running back to the past and does the exact opposite of the very ideas he’s teaching and embraces the whatever it takes idea. (Its a bit of a stretch but whatever).
This also explains why Hela could stop Thors hammer. I don’t believe she could wield it, she was just the extreme antithesis to the worthiness of Thor that she was the immovable force to his unstoppable object. She was without a shadow of a doubt certain of her supremecy, reveled in the death of the people, idolized conquest and control, and completely unwilling to sacrifice or compromise.
This is also why the other avengers could not wield Mjolnir. Tony is too prideful and believed his suit of armor was the greater good. He also could never turn on a loved one. (I would definitely believe he became worthy during Endgame but just didn’t consider it a possibility). Rhodey isn’t necessarily prideful but he does believe in the strict adherence to morals and law. (Again, Endgame would also make him worthy). My personal head canon is that maybe individually they couldn’t move it Endgame but together they could lol.
Bruce can not wield Mjolnir because he does not believe in sacrificing himself. Not in the sense of giving his life but in the sense that he would not sacrifice who he was to become Hulk, he would not sacrifice his mind and hated becoming hulk, often believing it was unnecessary and avoidable.
Clint could never turn on a loved one. Period.
I have a sneaking suspicion that Black Widow has been able to wield Mjolnir the whole time. She just never tried, being humble enough to not need to know, to spill her secrets and mistakes in winter soldier which is also when she puts people above government. In civil war tony claims double crossing is her nature, but that isn’t true. She honestly believes at first that the accords are the for the best allowing her to turn on steve and later tony. Finally Nat has always been willing to sacrifice her values and ideals ever since graduating the red room. That moment broke her and the reason it still effects her is because she realized she’d donanything for the mission, no matter the cost.
Just my thoughts on it.