I think it's the emotional attachment I talked about, you have two types of people who go to the theatre. One type are people who just go for the sake of liking movies and want to unwind. Within this category are some people that I know who are huge into the Oscars and they go to the theatre to watch the "best of" movies for when the Oscars are on. The second type are people who pinpoint certain movies, they go because a preview caught their eye, they're emotionally attached to the franchise/book, or anything else that garners their appeal. I would say this is more of the common movie watcher and why MCU is popular.
I don't know if it has anything to do with comic books, I know when I grew up the only comic book I enjoyed was Sonic the Hedgehog. Did not own one Spiderman, Batman, or Superman. However, the MCU caught my eye and I've enjoyed it ever since. For me personally, I think I care more for the acting than the actually main storyline. Of course I am a creative guy and I am more on that side of the spectrum in regards of I prefer more creativity than less. However, Robert Downey Jr. was amazing as Tony Stark, Hemsworth played a greek god pretty fantastic, Brie Larson was wonderful in her role, Black Panther was absolutely cast perfectly, and you can go on and on (Spiderman, Dr. Strange, GotG). I don't think you can say there has been a truly terrible acting performance by anyone in the MCU except perhaps Mark Ruffalo and even then he has a very small screen time and half of his character is CGI.
It comes down to the same properties of musical taste to be honest, person "A" doesn't get the appeal of Nirvana or Tool, while person "B" doesn't get the appeal of Cannibal Corpse or Dying Fetus. Some doesn't get why superhero movies are so popular while another bloke probably doesn't understand why a person out there doesn't like superhero movies.
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