Character Development Is Too Fast, And Everything Is Unearned
Remember how Steve Rogers did not become Captain America simply because he was a highly decorated soldier? Not only was he not the first choice for the Super Soldier Serum, but he was also considered to be a liability. He proved himself in spite of this. He tried to show that, in spite of his lack of size, he could handle whatever came his way. If nothing else, he was brave and heroic enough to sacrifice his own life to save his fellow man. That earned him praise, but even as a super soldier, he did not see the battlefield. Rogers also had to earn this as well. Eventually, he was trusted and proved himself as a hero.
Therefore, you can see how such a story of heroics would make it easy for Marvel fans to root for him. It’s likely difficult to do that with someone like, say, Loki. He spent most of his time in the MCU as a villain. Now we’re suddenly going to buy him as a hero who doesn’t just care about himself? What about some like Captain Marvel who seemed to earn her true captain ranking away from the MCU? We did not see her earn anything, which is why her story developed too fast. We did not get to see her prove herself on screen. There was nothing to care about, making it hard to root for her to succeed. See the difference between these people and Rogers?