Reason 1: The Message
A lot of Cap's appeal comes from the fact that he's a symbol of freedom and our great country. Some may argue our country is not so great. Cap is like the national role model, showing us what we could be if we're ever down on our luck. I mean he was created to boost our spirits as countrymen, 'America' is in his name for God's sake. If that isn't displaying the intended market I don't know what is. He's a strong-willed, morally righteous human being and at the end of the day all of us strive to be somewhat like that.
Reason 2: The Underdog Factor
Cap wasn't always the buff, shield-throwing patriot we know today. In his own words he was just a skinny kid from Brooklyn who 'depending on which comic' was born on the Fourth of July. A scrawny kid that just wanted to serve his country is a truly heartfelt plight considering he just wanted to follow his dreams. And yes, Cap took some 'Marvel steroids' known as the Super Soldier Serum but he's still an underdog. Think about this, you have Captain America who is a 95 year old man who granted has the physical condition of the best in any Olympic competition. That basically means nothing when your standing next to a God, a Hulk and genius billionaire playboy philanthropist Tony Stark. So a lot of people still underestimate Cap's value and in the movies as well as the box office he's proven his value more times over. Thor: The Dark World didn't do so hot (Fun Fact: Chris Evans even had a cameo in the movie) and fan-wise not all the Iron Man films are perfect. However, Cap is on his way to having a perfect CBM trilogy with Cap 3 on the way after Avengers AoU.
Reason 3: He's a War Vet
You should always respect those who fight for your freedom kids. Say your prayers, eat your vitamins, drink your milk yadda yadda yadda. Captain America was alive and fought in WW2, arguably the height of patriotism. He's still fighting for your freedom and he's only asking for 10 dollars to go see his movies which alot of CBM fans would gladly give up. Cap knows how to win and fight and has a strict moral compass that won't allow him to play for the wrong side. And believe me, whatever side Cap is on is the side you want to be on. Aside from being an Olympic-level athlete and having a vibranium shield, Cap is also a master tactician and strategist so he will without a doubt lead his men to victory. Having fought in countless battles and wars, it's next to impossible to outsmart him or put him in a situation where he wouldn't know what to do.
Reason 4: Quite the Rogues Gallery...
Just speaking as a comic book fan here, Captain America has some of the most creative and out-there villains I've ever seen. Armadillo, Arnim Zola, Red Skull, Crossbones, MODOK, Nuke the list goes on and on. Not a lot of people know some of his minor villains but even his minor villains are cool enough to be a main villain in this MCU. Cap faces Neo-Nazism, physical strength and even his own friend Bucky Barnes had to face Cap at one point, delving into the emotion-filled journey the Man out of Time has been through. Cap has died and come back so I doubt he'll be disappearing any time soon in the comics or the MCU. His status as an American figure will stand for some time and I hope we get as many Captain America movies as possible out of Chris Evans.Red Skull, one of the most vicious villains in the MCU should be expected to make a return at some point to the Captain America franchise.