The problem is Mackie and Stan are lesser actors than the one they are trying to replace. Neither have any charisma or personality. They are "lesser than" Chris Evans in every way that matters with the MCU main characters.

Captain America needs a big name, and these guys aren't the right caliber. MCU has lots of money, so there's just no excuse to take supporting characters and try and elevate them into the Avengers top circle.

Story was second tier as well - I hope this was just a blip on the MCU resume and not a foretelling of what's to come.