It's not about the quality of the film, it's about the lack of hype for the character. If people don't care to see the likes of Captain Marvel or even Ant-Man why would the care to see Sam Wilson just because he's called Captain America this time? Also it was made by the team behind Falcon & the Winter Soldier which featured the exact same character and people didn't really like that show (I did). And this whole think with the Hulk characters that they're using to attract audience is not going to work. The Incredible Hulk had been the only MCU box office bomb before Phase 4 happened.